The rate at which this website generates new posts, it is likely that older posts get forgotten rather easily ... notwithstanding the ethics of cross-posting.
If you are looking for a mobo for OC, then a good start should be this thread http://www.overclock.net/t/946407/amd-motherboards-vrm-info-database
I would also stay with AM3+ for reasons that I once knew, can't recall now.
Is there any disadvantage to using 4 sticks of 2GB DDR3 instead of 2 sticks of 4GB? Other than the physical distance from cpu and slightly higher power requirement, I don't see any obvious one. But I would verify it from experts. If you want to OC the DRAM (by changing FSB/SysClock), then maybe a higher speed DRAM is safer... something like DDR3-1866.
I believe the 990FX chipsets are the newest kind that allow you to use 2 graphics cards. The older ones can also support 2 graphics cards but don't expect their potentially combined bandwidth
(x16 + x16) to be available to the cpu in older chipsets. They will fall back to x16+x8 mode. With a single graphics card (x16) you describe, you do not need 990FX chipset. In US, I can get a very good AM3+ mobo with USB3, four to six SATA 6Gb/s ports, etc. for below 100 dollars. In euro, it should be a tad cheaper. However cheap you go, make sure your southbridge is at least SB750 or better ... if you want any kind of high throughput on the I/O side.
Regarding the power supply... if you add a couple of rotating spindle disks, a moderately powerful GPU of Radeon 6000 series, and an overclocked CPU with 4 sticks of RAM, you may be on the borderline with a 450W PSU. In theory, I don't think it is a problem. But looking at PSU reviews at hardwarecanucks.com, legitreviews.com, etc. I see that unless it is has certified level (like bronze, silver or gold), it is unlikely to provide all the power when the load is on the extreme. Exceptions do exist however, I myself would prefer to stay with bronze certified PS from Antec, Corsair, Silverstone, OCZ/PC-Power-and-Cooling, high-end Roswill Capstone series, etc. I once tried to OC a Core 2 Quad Q9550 with 4 HDDS and a small graphics card by an Apevia 450 watt and it took me 3 months to realize the root cause of sudden system hangs. After changing to a 750W PC Power and Cooling, all my instabilities went away. You can try with the PS you have but be ready to buy a much bigger uncertified one or a 500W bronze certified one down the road.
Regarding GPUs... note that Radeon 7000 series uses PCI express 3.0 and I don't think AMD chipsets support that yet. A 7000 will work on PCI express 2.0 slot but at lesser bandwidth.
Have you taken a look at frostytech.com for cooling solutions? FX-6300 can warm up very quickly under Prime95.
By the way, the Q9550 you have in your profile can easily go to 4GHz -- which I do 24/7 on a GB EP45-UD3R mobo. It is still a very potent chip at 4 GHz and does all 720p transcoding with ease.
Hope it helps.