all ram is starting to go up. i'd be surprised for it to go much higher than it is, now. there hasn't been much innovation in the field in about six months, with pretty much no new packs coming out. i'd be surprised if that held true over the next six months as well.
the 9800GTX+ would be enough for you, then, as it's particularly strong with GPGPU and CUDA technologies. you could go lower and not be hurt by it, but don't go too low - a 9500GT (512mb of ram, 256 or 128 bit bus) or the 9600GSO would be the lower end of the spectrum for what you'd want, i'd think. if you're not using GPGPU- and CUDA-enabled applications...you will be, in a year or two. so get a gpu that's worth the time.
it's quite expensive, yes. the days of spending 600$ on a computer and having it be good for longer than six months are gone, though, with the astronomical rise in the renewal speed of technology. go big now, so that you don't have to spend money for a long time. and always - ALWAYS - spend more to get the warranty. it's worth it, in the end.