Free software tweaks and tips
Deciding what component to upgrade can be tough, but here are a few tips. First, definitely upgrade your ram if you don't have at least 512 megabytes. If your main concern is speeding up renders in an application like 3ds max then upgrade the CPU.
If realtime operations like games are slowing you down then the decision is more difficult. To determine if the CPU or video card is a significant performance bottleneck you'll need to run some benchmarks. The key to benchmarking is understanding what the numbers are telling you.
First, lower the benchmark resolution to 640x480. Using a low resolution like this will reduce the impact of the video card on performance. If a game is unplayable at 640x480 you might need a new CPU and video card, but you don't have enough information to know yet.
Now run the benchmark at 1024x768 resolution. Compare the results between resolutions. If the numbers are fairly close you probably need to upgrade the CPU because it isn't fast enough to stress the video card. If the performance drops significantly as you increase resolution a new video card will help. If the numbers scale well between resolutions upgrading either component will help and it will be hard to tell which component will have more of an impact.
Testing with other resolutions will give you more data to work with. Just remember the basics. High resolutions tax the video card more than low resolutions, so low resolutions usually isolate the CPU.
Ars Technica - there is even an article on tweaking Windows NT 4.
Outer Technologies - the creators of Cacheman.
Tweak3d.net - has a fairly comprehensive list of tweak guides.
Black Viper - details and tips for optimizing your Windows services
3D Benchmarking Links
3DMark - benchmarking software.
Tom's Hardware has an informative article on accurate 3D benchmarks. Find out what really matters and why some benchmarks are better than others.