This seems to be a hot, constant debate here on OCN. Let's iron some of these arguments out.
First and foremost, the 9800 GTX is not an upgrade. It performs right about at the level of an 8800 GTX, if not a tad worse. This is a "GTX" branded card on the high-end of the mid-range for today's standards.
You can find benchmark results from quite a few reputable review sites, but keep in mind that they are using a processor and motherboard that isn't in common usage yet, such as the QX9650, so their results are a bit above what most of us will experience.
These reputable sites also tend to stack quite a list of games for their reviews, and consistent results are hard to come by. All games are optimized differently, so your results will always vary in the sense that sometimes the 3870 X2 or the 8800 Ultra will come out on top, leaving the 9800 GTX in 2nd or even third place.
Keep in mind the price. This should be a clear indication of what you are dealing with. For a long time the 8800 GTX stood firm at about $550, and we were all so very satisfied by its performance. The 9800 GTX is a humble $330. I paid $350 for my 8800 GTS 512 when it came out, and, well, look at my sig. You are looking at 3 elements: the depreciated value of a monster video card that has been, for the most part, overcome by the GTS 512, the GTS 512 causing this rapid depreciation, and the 9800 GTX performing in between the GTS 512 and the 8800 GTX. The price is very fitting. No need to upgrade to the 9800 GTX from a GTS 512, if anyone is considering this move. They are pretty much on in the same in terms of performance. Again, every game is handled differently, so results fluctuate from game to game.
Though it is not a topic in this thread, it is worth repeating. The 9800 GX2 is in fact a good upgrade from even an 8800 Ultra. I haven't seen any results for a single GX2 vs 2 Ultras in SLI, so I can't say how far the GX2 goes but I can confidently assume that a single GX2 can beat an Ultra SLI setup. By how much, I can't be certain but I'm sure it's possible, if not close. Cool thing about that is you don't have to have an SLI motherboard, you don't have to have a PSU with 4 6-pin connectors, and you leave room in your case for airflow. As most know, SLI is a bit of a silly concept, seeing as the scaling is just awful and you still have to pay 2x as much for a 20% increase. Also consider that 2 8800 Ultras would cost you about $800, whereas a single GX2 is less than $600. Price/performance ratio is apparent here.
Oh yeah I know this is a long post but I am compelled to share my opinion. Ok, there's another part of the picture to look at and that's the raw numbers. Some look at the specs of a GX2 and say "wait, it's 512mb x 2 on a 256-bit bus?". Well, it would seem that these numbers aren't a clear indication of performance. Look at the numbers that the 3870 X2 boast and it still falls short. Bottom line here is to check out the multiple benchmarks available to get an idea of what sort of performance it offers.
Be advised, the 9800 cards are going to replaced this year by the 9900 GTX and GX2, with completely new cores. Those will undoubtedly bring some serious gaming results, and we are all anxious to see how they stack up against the current, controversial 9800 GTX and GX2.