You need to check your motherboard manual to see the type of memory you can install, you also need to check how many spare slots you have. Memory has never been cheaper 128Mb of
PC133 SDRAM will cost you no more than 15 quid.
As for the GFX card if you've got an AGP slot you'll be able to get a Nvidia GeForce2 chipset card with 32mb for about £50. It all depends how much you want to spend really.