Hi,
I can see you are in the UK, try novatech.co.uk, or ebuyer.co.uk, or scan.com, they all have really good prices and some really good kit to upgrade your system with.
As everyone else has said so far the sims 2 is a BIG game, that take loads of memory to run well, the MINIMUM spec for this game is 32mb video card, and an 800mhz processor, with 256 mb of RAM, you only really just make this, which is why your game runs slow.
When a game says minimum requirements, they normally run with these but you could do with at least half as much again as they suggest for it to run at an acceptable level.
Hope this helps.
Rick.