We don't think that you need to be spending a great deal on a CPU right now, if you are planning to upgrade to SLI in six months time. You may as well buy a faster CPU then, as there are likely to be faster CPUs available, which means that the current CPUs will drop in price. You could get away with a 3000+ and GeForce 6800 GT for now, and still be able to experience something similar to what can be experienced on an Athlon 64 FX-55, at less than a quarter of the price.
In six months time, you can find a second GeForce 6800 GT, and maybe upgrade your CPU. Alternatively, you can upgrade your cooling solution, buy some faster memory and overclock your CPU to give you the necessary kick in performance that will allow you to experience two GeForce 6800 GT's in all their glory.
There are many options, and it is all about thinking things through carefully if you are planning a two-phase upgrade to SLI. If you think your upgrade path through carefully right now, you could be saving yourself a lot of money when it comes to completing the upgrade to SLI in the coming months.