Personally I'd agree with the people here and say just tough it out with the 6800GT (which is a great performer given it's age). I was just trying to give you a measure of performance gain you'd get for your 200-250$.
Obviously everyone in here thinks that for some weird reason, actual gaming benchmarks are useless semantics when measuring performance, and the 7800GS, a card based on the 7800GT core, is basically the same thing as a card based on the 6800GT core. Ok, I wont argue. I wont try to find any other gaming benchmarks either, since all those will be nothing but pointless statistics. In fact, I guess it's safe to say that any video card review is nothign but a collection of pointless numbers and statistics. As if these reviewer fools think that people actually look at these numbers to see if there's a relative performance gain compared to their current hardware ... pffft. The main factors that should determine your video card purchases are
a) The hot animated chick on the box
b) The amount of times the words "extreme", "performance" and "PCI Express" are used in the description of the card on the back cover.
All jokes aside though, I've owned a 7600GT (which is somewhat weaker than a 7800GS) and I know for a fact that I've been getting much better frame rates in FEAR, COH and currently Medieval 2 : TW than my friend's nearly identical computer (based on an OC'd d805) but sporting an overclocked 6800XT instead of a 7600GT. I would say gameplay was much smoother at 1280*1024 at high/medium settings on my rig, than it was at 1024*768 at mediium/low settings on his. In my understanding the 6800XT is somewhat weaker than a vanilla 6800 - but considering that he's OC'ing it to the max, it can't be THAT much weaker.
Again, given all that I've mentionned above, I still consider that a 250$ would best serve you as a "downpayment" on a new PCI-E core2 system, that it would on a 7800GS upgrade.
As for whoever mentionned that Tom's has different cards running on different hardware, that would make their entire benchmark collection a little absurd, don't you think? I think it's a safe bet they tried to keep the base testing platform identical for all the card (obviously they had to find another mobo for the agp card). In fact, the CPU in all cases is an FX-60.
p.s. OC'ing your card, especially a 6xxx/7xxx series Nvidia card is always a good idea. It's almost always a guaranteed 10% free performance boost on stock cooling. Go for it man, see if it can satisfy you while you wait to build a new system.