Ok lets run through it.
DDR3 on socket 775 = don't bother, running the memory faster than a 1:1 raito on a socket 775 has little effect.
Remember the actual base speed things are running at are, memory = listed speed / 2, CPU Listed FSB / 4.
So Q6600 listed 1066 MHz FSB so actual speed is 266
say DDR2 PC2-6400 listed 800mhz so actual speed is 400
So running both at stock means a memory ratio of 2:3 aka 1:1.5
The reason a scoket 775 system get little benefit fromrunning the memory faster than the cpu is that the data is going between them via the north bridge which is running at the base fsb.
In the above case the while the memory is running at 400mhz the cpu can only talk to it at 266mhz
The reason you do get a very small performance benefit from running the memory faster than the cpu is down to latinces, basicly the faster and lower the timeings on the memory the shorter it's cycles so quicker it can respond to call on it.
However these wait cycles are very short compaired to the speed at which the data can be sent to and from the memory, which is why running the memory faster than the base fsb give little in the way of a performance boost.
A simular situation for the memory sticks you use, 2x2gb is better than 1x4gb because it can be run in dual channel alowing both the memory stick to be talked to at the same time. (also 2x2gb sticks are generally cheaper than 1x4gb)
On to what you should be looking at.
a P45 or P43 chipset motherboard, these will be your best bet.
P45 = not very cheap
http://www.scan.co.uk/Products/Asus-...-SATA-RAID-ATX
P43 better price
http://www.scan.co.uk/Products/ASUS-...TA-II-RAID-ATX
EDIT, just spotted 900USD so I take it you're USA based not UK so let's have a quick look at newegg
http://www.newegg.com/Product/Produc...82E16813128414
http://www.newegg.com/Product/Produc...82E16813131377
RAM, ddr2 pc2-6400 is the sweet spot for a q6600 gives you a theoretical overclock of 3.6ghz at 1:1 ratio ie before you even have to start overclocking the memory.
Ideally CAS 4 or 5 (lower is better, however as above it doesn't have much effect on a socket 775 system)
For lower costs it's worth looking 2nd hand, you may be able to pick up a 4x1gb kit fairly cheaply.
PSU, DON'T skimp on the PSU, don't need to buy something massively expensive or super high wattage, something in the 450-550watt range is fine as long as it's a decent unit.
Corsair, Seasonic, Enermax, OCZ, Antec (not Basiq model) Xigmatek and silverstone are decent too.
Although it's harder to call here, as there's a load of PSU in the US that never come across this side of the pond eg PCPower&cooling (although I know they went under)
Ooo just spotted this
http://www.newegg.com/Product/Produc...82E16817341016
OCZ modular 500w after rebate $40
nice price.