Tiff, on the back of your pc will be a serial number, if you enter this into compaqs website it will tell you exactly what hardware is in your machine, so u jot down the memory type and and just order the same.

Any current agp card will be ok in your system, no point spending a lot on one, i would personally get a pny Geforce 5700 for £50 and your machine should take ddr 2100 ram, add another stick of 512mb crucial ram for £50 and it means your pc will play most games, maybe not at 700bhp spec but will still be ok and you aint shelling out big style either. Then if you get hooked on the gaming as rudey said a new machine will be in order.