8800 gt or 9800 gt performance ??????

nothing for now, but later on 9800gt's will support tri-sli and be made on 55nm which will decrease temps.

hope this helped.

Welcome to the fourms!

There is no performance difference between stock 8800GT and 9800GT's.
They both use the same G92(b) chip (the "b" version being a 55nm shrink of the G92) with the same features and clocks.
IF the 9800GT has a G92b chip (no real way to tell as it uses both versions right now) it will use a little less power, run a little cooler and possibly overclock further.

As your Intel based Mb has no SLI support, you do not need to worry about having possible tri-SLI support (only some of the 9800GT's have it anyways).
Likewise, unless you are gaming above 1920x1200 (which you are not, especially with a midrange card) you will not need more than 512mb of memory.

If these are your only choices, get the cheapest you can locate unless you locate one with dual slot cooling or one that is overclocked for just a few $ more.
If your choices are a little more flexible, try and save a little more cash for a much better performing 4850.