To reiterate others' comments, the 7800GTX will make a noticeable difference on FEAR (and probably other upcoming games) even at lower resolutions. My x850xtpe did not play the FEAR demo all that smoothly, even at medium settings at 1024x768 (with AA/AF). It was OK, but not silky smooth. The 7800GTX would probably also make a noticeable difference in BF2 at 1280x1024. Your choice really depends on your specific sensitivity to price. In my opinion the 7800GTX definitely justifies a $100-150 premium over all of last year's cards. Whether it justifies a $151+ premium is debatable. The 7800GT might be a good compromise option, assuming you can find it for $400 or less when it is released.
Sounds like you plan to stick with nvidia this round, but there have been some recent deals on the ATI x800xt - x850xt pe class cards for around $230-280 (e.g., a refurb from the ATI store). These probably have the best price/performance of the higher-end cards right now (and also the X800XL to some extent). If you can find a similarly-priced 6800GT, then go with that, but I haven't seen any at those prices. If you can find one for ~$250, it's touch to justify paying double for the 7800GTX.
In any event, you should probably wait just a few days until the 7800GT is released and prices on all cards have a chance to drop a little. Or wait a little longer until the R520 is released, for the same reason.