That CPU is plentiful for NT6.
I assume this is a desktop and not a laptop right (Crucial UK say its a desktop, emachines site is about as much use as a beergarden). If so, your system takes DDR2 5300 RAM upto a maximum of 1GB per socket (2GB max), and it should cost about £20 for 2GB RAM or £12 for 1GB RAM.
1GB - http://www.ebuyer.com/product/115961
2GB - http://www.ebuyer.com/product/116260
It is pretty straight-forward to upgrade memory yourself, there is guides on the net, only takes a few minutes, but I can do it for you for free if you get stuck.
If you upgrade the memory, you will feel the difference right away, and even if you only add 1GB, it will be a massive improvement and make Vista run like it should do (even Vista Basic will struggle like hell with 512MB RAM)!!