You want a faster system? Put faster parts in it. That's the simple an­­swer to a question that every PC owner asks from time to time. But replacement parts aren't free, and cash-strapped computer enthusiasts know that the key is to put their money where it counts most.

That's why the labs sought to identify which upgrades give PCs the best performance bang for the buck.

First, we separated our benchmark tests into two components: general system tasks (including office applications, photo editing and movie encoding), and gaming. Then we divided our up­­grades into four categories: CPU, RAM, hard drive and graphics board.

We selected two primary test systems to represent the kinds of desktop PCs that users are likely to want to overhaul with hardware upgrades: a three-year-old Polywell with a 3.4GHz Pentium D processor, 2GB of RAM, a 500GB hard drive and a GeForce 8800GT graphics card; and a one-year old Dell with a 2.8­GHz Core i7 CPU, 4GB of RAM, a 1TB hard drive and an ATI HD 5670 graphics card.

We then ran tests on the systems using various combinations of the above upgrades to determine which configurations yielded the best return on investment. The results for individual PCs will vary greatly, but the data supports some general conclusions about which up­­grades make the most sense, and our recommendations may surprise you.

Upgrading the CPU

Bumping our Polywell's processor from a Pentium D to a Core 2-class chip yielded instant and obvious performance improvements across the board. Moving to a 2.67GHz Core 2 Quad prompted a 36.8 percent jump in performance on general apps. Using an older 3.0GHz Core 2 Duo was even more effective, with a boost of 52.6 percent, probably due to the speedier frontside bus in the Core 2 Duo over the Core 2 Quad. Graphics performance improved even more for both upgrades.

Best of all, our CPU upgrades were affordable. The Core 2 Duo upgrade rated as one of our best values in the entire study, costing about £0.60 for each percentage point of general performance improvement.

Upgrading RAM

Conventional wisdom has always held that upgrading your system's RAM will give it an instant boost. The upgrade is easy to perform, and it makes sense because RAM is cheap. But if your PC already has even a moderate amount of RAM, you likely won't see much of a speed increase from adding more. For example, when we bumped our 2GB system up to 4GB, we got a paltry 1.3 percent improvement on general apps and virtually no improvement on games. Similarly, our year-old Dell's performance im­­proved by just 3 percent when we moved from 4GB of RAM to 8GB.

The limited benefit that the up­­grade provided in our tests made investing in more memory almost pointless.

Upgrading the hard drive

Solid-state-drive technology promises a dramatic decrease in hard drive latency. And in our tests, moving from a 7200-rpm, 500GB traditional hard disk drive to a 120GB SSD resulted in a 8.0 percent boost on general apps and an 18.4 percent speed jump on gaming.

SSDs aren't cheap, and you lose a large amount (nearly 75 percent, in our case) of your storage capacity in the bargain. Still, presented with prices of £16 for each percentage point of general performance improvement and £7 for each percentage point of graphics im­­provement, power users may find the outlay worth their while.

Upgrading the graphics board

No mystery here. Up­­grading to newer graphics will do wonders for your gaming. When we upgraded our old Polywell to an ATI Radeon HD6870 card, gaming performance improved 14.9 percent. With our newer Dell, an ATI Radeon HD6850 gave us a 117.2 percent boost in gaming on the machine. But neither improved general application performance.

Mileage varies a bit. The Polywell's cost of £9.20 for each percentage point improvement is high, but the Dell's extremely low £0.94 for each percentage point of gain makes the graphics board upgrade on that PC the most cost effective upgrade in our roundup.

Multiple upgrades

You're likely to fare even better if you upgrade components in combination. Performing all four of the upgrades on our list, CPU, RAM, hard drive and graphics board, on our old Polywell desktop improved its system speed by 67.1 percent and boosted its gaming performance by 166.3 percent.

We also spent more, but the overall improvement was far greater than the sum of the improvements from the individual upgrades. Ultimately we spent £6.26 for each percentage point of general performance improvement, making the four component upgrade a surprisingly reasonable bargain.

You don't have to upgrade everything to see a boost, of course, your best bet is to focus on performance bottlenecks. To find them, visit the Windows Performance Information and Tools Control Panel. Focus on the lowest numbers listed in the panel's Windows Experience Index and upgrade accordingly.

Forklift upgrade

Does it make sense to perform a bunch of upgrades when you could simply buy a new PC? Even under the best conditions, upgrading is a hassle, and it gets expensive: Depending on the CPU, we spent about £450-£500 to buy the components for our Polywell upgrade, more than some new PCs cost.

Effort and risks aside, it still makes sense to upgrade in some instances. Graphics are a sore spot here, as new computers with integrated graphics fared extremely poorly in our gaming benchmarks. If you want better game performance than your current system provides, focusing on a new graphics card makes more sense than buying a new rig that uses integrated graphics.

General apps were a different story. We had to spend £520 on an overclocked 3.3GHz Core i5 PC with 4GB of DDR3 RAM and a 10,000-rpm hard drive to substantially improve on the gains we saw from our CPU upgrades alone. In that case, in­­vesting in a new PC would have made more sense, but for almost everything else, selective upgrading would have been the wisest choice.