A Q9550, an X48 board and 2x2Gb DDR3 would probably be your best bet for future proofing. Get memory that will hopefully allow overclocking with Nehelem later on and get something that's easy to find since Nehelem will probably support triple channel memory (just buy another identical 2Gb stick later on).
The Q9550 should be really fast for quite a while if you can get it to the 4Ghz range with a good cooler. But if Nehelem is insanely fast and is a must-have it'd be good to have the DDR3 already.
... then again, DDR3 prices will be coming down so it may actually be cheaper to get DDR2 now, sell it later on and get cheaper, more mature DDR3.