Help! Which graphics card?

OK... I have hopes that I will be upgrading soon. It's obvious that I'll make the move to a nice dual core processor, sneaking up on twice the clock rate of the Socket A processor of my current computer, and a lovely SATA II hard drive (not quite hard core enough to consider RAID... yet) and plenty of DDR2 RAM.

The question I have, though, is what graphics card to get--where the sweet spot is for price/performance. I really wish that work were far enough along on Open Source 3D-capable ATI device drivers that I could make the switch, but for the time being at least it's got to be nVidia. The only 3D-intensive thing I do is run the SL client, though perhaps that will change if I have sufficient graphics capability to turn all the knobs up on Compiz Fusion. (I hasten to add that Compiz Fusion can do amazing things with far more modest requirements than the bloated obscenity that is Aero.)

So, what do you use, and how do you like it? Can you run the sliders for the Windlight client up against the wall and still get a good frame rate? I know that the upper bound for me on price will be something under $200, since the widely-acclaimed 9600GT cards are going for that and I'm sure it will run SL beautifully, but something less may do well enough for significant savings. Let me know what you've found.

P.S. I am a little undecided about CPU still--there's a 2.8 GHz Athlon 64 X2 with 512K L2 cache per core, and then there's a 2.6 GHz Athlon 64 X2 with 1M L2 cache per core. Is locality of reference enough that the added cache beats 200 MHz difference in clock rate?

P.P.S. Should I take the plunge and go for PCI Express 2.0? I'd have to to get the full advantage of the 9600GT.