what do you want the new card to do.do you want to play the latest games or do you just want something a bit better than what you have at present?
does your motherboard have an agp slot or do you need a pci card?
aida32(any version) will tell you the make and model of motherboard you have and the slots available to upgrade the graphics click here you may also wish to check the type of agp slot you have if that is what you have(almost all fairly recient mobos have these) click here
something half decent will cost about £100 - £150 or so.top notch cards cost over £200 some close to £350.all depends what you want it to do and the rest of your system is also fairly quick.no point in putting a monster graphics card in a very old system.
some reviews may help click here