If you are dying to get a new graphic card for running your PC (since your old 9800 already blown out to heaven ^^), then try to look for any cheap AGP card available on your market. Better save your money first, since there are about many new product launch by this year I believe.
In my place here we still have like ATI's or NVIDIA's AGP for less then 80$, hopefully you will also find one.
Why I'm suggesting to get cheap AGP card and save your money first are:
1. You should put a lot of investment since you should replace processor and mobos as well. I assume you will also have to replace RAM (are you using DDR2 or DDR1?).
2. In the current situation is better wait for couple of months: chipset X48 are about to launch from Intel, AM2+ socket mobo's start to launch from AMD with chipset 770/790X/790FX but still not yet convincing enought to buy. ATi and NVIDIA are about to launch a new graphic card I believe
All of these things makes quiet complicated for your situation, so better wait than sorry
However if you are really can not wait for an upgrade, AMD 790X chipset seems the better choice for you, I believe we can mobos + Athlon 64x2 combination for less then 250$ or even under 200$ if you get lower Athlon speed. But for VGA will be better to get ATi AMD as well. 3870 are good choice and have a competitive performance to 8800GT with less price (less than 200$ soon).
For SLI, you don't really need it unles you are planning to build multi GPU system. Which again if you are planning for multi GPU, then you have to spend some extra money. You can really see multi GPU benefit if you are running in 1900+ reso or 21"+ monitor. I'm using 19" monitor, with 1440 resolutions with HD3870 and I can play all latest game with max setting (except for Gears of War and Crysis, I prefer to scale it down to medium due to a little bit lag).
If you can tell us more detail of what you are up to do with your PC (e.g for gaming, or for graphic designs, or whatever), and also your current system and your budget we can tell you more details
GL anyway