agp 2.0 limits u to not very good cards. that 7800gs is 8x speed but ur slot can only go up to 2x. that means ur not getting nearly the bandwidth speed that the 7800 can give u, therefore a lower card would give u very similar performance.
u can get that card for sure but im not sure it would be worth the money. i suggest either u get a stick of 512MB ddr-400 kingston ram and that card or u completely revamp ur computer if u have the money.
pci-e are the new video card slots with 16x bandwidth which gives much better performance than agp and u can get a good motherboard with pci-e and the same socket as ur celeron (im guessing 478 or 775, the two newest). pci-e cards tend to be a few dollars (or pounds) cheaper than the same version in agp. plus pci-e will give u room to expand later on
although if ur celeron is a socket 478 (u can find this info on the first page of cpu-z under processor beside package) u'll have trouble finding a mobo (motherboard) with a pci-e slot