The X700 pro is about equal to a 6600GT...a $100 (USD) AGP card. For $90 Canadian, that is a very good price. But if you have a high end system and hope to play the latest games at a higher resolution with more details, then the 7800GS is the one to get. Keep in mind, that the 7800GS is about like a $125 (USD) PCI-e card (7600GT, X850XT) and nowhere's near a $200 PCI-e card or the upcoming X1950 pro AGP.
Anyway, to give the best answer we need to know:
1) your system specs
2) the resolution you game at
3) what games you play
With a P4 2.4GHz and a FX5600, I would go for the X700 pro. 7800GS simply isn't worth that much extra. If you had an A64 or P4 3.2GHz and a 19" LCD with native 1280x1024 resolution, then the 7800GS would be better, although really Oblivion would still hammer that card bigtime.
With your gaming resolution the 7800GS was the better choice. You may find out you are CPU bottlenecked in many games, and a good CPU overclock will help.
Is that a P4B(533 bus) or a P4C(800 bus). What motherboard? Dual channel Ram? Reason I ask is a i865pe P4 2.4C running dual channel is quite a bit faster than a P4B running single channel mem. Both will struggle in some games.