I take it you're using 9800GX2's now? Are you using one or two? The 9800GX2 was a good benchmarking card, but if hit the wall hard when it came to gaming and didn't hold a candle to the GTX 280. The 9800GX2 was essentially two downclocked SLI 8800GTS 512's in SLI using the same PCIe slot. The problem with the G92 refresh cards was that they had a gimped memory subsystem (GDD3 + 256Mbit bus = low bandwidth) and the problem was exacerbated when you put them in SLI (essentially a 9800GX2). The GTX 280 took care of this problem with a 512Mbit bus and faster RAM, not to mention a beefed up core. At lower resolutions (1280x1024, 1680x1050) and in non-memory intensive games/apps with good SLI scaling, the 9800GX2 really shines. However, once memory becomes an issue or SLI scaling isn't perfect, it will fall hard and fast compared to the GTX 280 single care solution.
All of that said, GTX 280's are really coming down in price (<$300 with a sale), probably because new cards are right around the corner. However, unless your 9800GX2 is really is hampering your gaming experience (or whatever you use it for) I would recommend holding off for this new refresh. This new generation is on a 55nm process and should run much cooler, overclock better, and use less power than the current GTX series.