I expect that you are running an AGP motherboard.
PCI express has been out for at least a year now and as a result, the fastest and the newest gfx cards are aimed toward PCI express.
In other words AGP is slowly dying.
Nvidia have reeningineered some of their cards, notably the 7800 for AGP and no doubt there is still some life in AGP yet as ATi do the same.
I would not consider spending very large amounts for AGP cards for this reason, its likely you will be replacing your system in the next few years and unfortunately you will not be able to carry all that much from the old one to the new one, which matters if you are trying to do this on a tight budget.
You might need to consider your power supply too, some of the later AGP cards consume lots of juice, and if your system is already loaded up, two or three hard drives, lots of banks of memory, Pentium 4 processor etc, thre is a goddly chance you will need another power supply.
I think you would do well getting a Nvidia 6600 oc card, this is a card which is already slightly overclocked, not by much actually, but it has the manufacturers warranty and gives you just a little bit more peformance.
Another good option would be to go the EBAY route, for the same money you’ll get yourself a 6800, but you have to be slightly careful of which version you get, and also these are power hungry, so you may need to think about your power supply too.
I’ve seen plenty of horror stories about the Leadtek ones, to the extent that I would not recommend one.
Don’t bother with the Geforce 6800LE versions, they have a reduced number of pipeline - sometimes they can be activated but for the extra cof maybe $40 you can get the real thing anyway, and you will not miss the money 12 months from now anyway.
The Geforce 6800 GT versions are good, better yet are the Ultra versions, you will pay for the latter though, Asus do a sort of halfway version, its really a GT card with selected GPUs and faser memory the V9999, not bad but they only have 128m memory and nowadays the standard at this level of card is 256m.
The Geforce 7800 range of cards is quite new, they are not massively better than the 6800 so I can’t see much reason to shell out for one, unless you have an application that really needs on, there are not too many second hand ones around, so you’ll have to buy one new.
I have read that the XFX cards are the ones that are best to go for.
All I know about the ATi cards is that they use less power, no doubt they do some very nice ones but I haven’t had any of their recent offerings.