a 7600 is a good card, they do go for around £150, but i've not seen an agp one yet, only pci express, so you need to find out exactly what graphics card slot your new pc will have before buying a card.

i prefer ati cards over nvidias, and if i was buying a new card now, i'd buy an ati1800xt, but they over £200, and you'd also need a new psu in your pc, as they need quite a bit of power.

one other upgrade for your pc at a later date would be to get another stick of 512Mb of memory