When do you plan to finish building your computer? Why I ask is that I highly recommend AGAINST buying parts ahead of time just because you have the money now. Save it. If you're afraid you'll spend it, put it into a CD at a bank. Here are a few reasons to save the money until you can buy all at once:
1) You won't know if a part is DOA until your other parts comes in (yes DOA happens).
2) Warranty starts the day your order is placed (invoice date) so if you buy some parts today and get your last part 1/2 year from now, you've just burned 6 months of warranty for nothing.
3) With some exceptions (such as RAM price fluctuations) generally speaking you'll be paying more now than if you wait. I helped a business with 10 new machines, and they wanted to buy/build them over a month before they needed them because the money was there. I convinced them to wait, and three weeks later got them a better system at a savings of $140 per unit ($1400 saved). This is an extreme example because of a bit of luck with some hot deals, but I would have been suprised if I had not saved at least a few bucks.
4) New stuff is always coming out, and in a few months when you're ready to finish building, you may be saying to yourself, "duh, I should have waited because this new model came out."
With that said, I ask the same thing as cmrmrc, "are you planning to overclock?" CPU arguments aside, these days there is little functional and performance difference between motherboards, except often the more expensive ones can overclock better.
Another thing I would ask is for you to think about your upgrade cycle. If you are the type to buy a system and use it to death, and then 2-4 years later buy a whole new system, then upgradability won't matter. If you like to refresh your system every 6 months or so, then choose parts more carefully, especially the motherboard.
One last thing is that gaming is more dependant on video card than CPU. As long as you have enough CPU, then spend every last cent on video card. For instance if you budgeted $350 for CPU/mobo, but then only have enough video card budget for a $150 7600GT, then you're short changing yourself as a gamer. That $500 would be better split at $250/$250 because $250 can get you a 7950GT, which is a sight better than the 7600GT for gaming, and $250 can still get you a dual core overclockable platform (albeit an AMD one).
For your budget and buying today, and assuming some overclockability, hardwareking and Chenchu have some good suggestions. Those motherboards are near identical, with the more expensive one (by around $30-40) having RAID and "better" capacitors.
If you're waiting until you have Christmas money in hand before you can buy the rest of the parts, then WAIT. Why? Supposedly sometime in January Intel will release the Core 2 Duo E4300. Not only will that model be a bit cheaper than the E6300 series, it'll be potentially easier to overclock using cheaper motherboards (due to low initial FSB and high multiplier).