For the price range you're going to struggle to improve it at all to be honest.
Without looking (so this might be completely wrong) the X700 will have more up-to-date features and extensions for DirectX9 compatability, but the core of the card won't have the raw power to actually make use of them, so it might even slow your games down, should a game not allow you to turn off - for instance vertex shaders can't usually be turned off.
In my honest opinion, you'd get more advantage from actually getting an AGP/PCI-E motherboard, such as the fantastic ASrock 939dual-SATA2 (ULi M1695) (it's about £45), which will let you upgrade to a better graphics card when you want it, you'll be able to move up to the Athlon64, and keep your new RAM. If you absolutely need a bit of a boost in performance, that'll probably work better, and open you up to the PCI-E range of graphics cards, which are both cheaper, and usually faster.