Hi Captain Spencer.
In order to make a significant difference, you'd have to upgrade your video cards. Almost certainly that would mean an upgraded power supply, since most of the top-notch cards need extra power suplies and your existing one most likely won't have th eright plugs.
Your processor would need to be much faster. That would require a new motherboard most likely.
Then, you'll need new RAM. 4 GB would be best, but 3GB as a minimum.
That means you can keep your existing mouse, keyboard and probably the PC case, although some of the newer motherboards have a different form factor. Possibly your existing monitor would be reusable.
The cost of even half of the above items as components is much more than a new PC...
I'd suggest that you go to the Novatech site and have a lpoke around there. I get the vast majority of our hardware from them (have done since they first opened as a two-man company in a small "shed" in Southsea, 18 years ago or therabouts).
Good prices, most stuff is ex-stock and you can go and collect within an hour of placing your order or get it delivered by any of half a dozen different services.
www.novatech.co.uk