ummm... i don't know how to overclock. But i think i can learn
OK, I just assumed it because of the 3.4 GHz...
Core 2 Duo E6400 3.4 ghz
Geforce 7600GT 256 mb
2x512mb RAM DDR2
If you take overclocking out of the picture, then just about any name-brand motherboard will work. I recommend one based on the P965 Intel northbridge, (like the Gigabytes above). You also will have an easier time of it by using 1.8 volt DDR2/533 value ram such as this:
By Q4 2008 DDR2-667 will be standard, and DDR2-800 will be commonplace (and it'll be standard voltage DDR2-800 too).
Since 1333 FSB will be 'the norm' come Q4 2008, I would recommend Dual-Channel DDR2-667 now, using 2 x 1 GB sticks, then adding another 2 x 1 GB sticks (idential if possible, so Kingston recommended here because of the massive time delay on the planned upgrade).
We all might be, but the 'Original Poster' doesn't seam as likely to change plans, especially it they only want to allocate finite funds to their project.
They've made a smart move IMHO since they'll get good longetivity and cost effective upgrades out of a Core 2 Duo system.
Their best option would be a Core 2 Duo E6400 (2.13 GHz), but on a 1333 FSB, to overclock to 2.67 GHz, without needing to worry about other parts running out of spec.
2 x 1 GB DIMMs of DDR2-667 (aka: PC2-5300/5400) will assist with that 'overclock' (of CPU only), and ensure they're ready for 1333 FSB 'standard' parts around Q4 2008 - when they'll be much cheaper.
ED: 2 GB of RAM (or 2048 MB) is what Dell are recommending (and everyone else) as the 'heavily suggested' quantity of RAM to run Windows Vista x64 editions, in 2 years time that is most likely what many consumers will want to be running (if not much sooner than that).