The Gigabyte GA-M52L-S3P only supports 95W TDP and under AM3 chips http://www.gigabyte.com.tw/Support/Motherboard/CPUSuppo...
even so it's essentially an AM2 on rather dated HT 1.0/PCIex16 rather than HT 3.0/PCIex16 2.0 and this is outside the IGP performance which has greatly picked up after AM2 chipsets Lastly Antec/Corsair/OCZ PSU
1. The sun is going down on DDR2 platforms so when the music over at least one could reuse those DDR3 ^^
2. Yes and the HD 4870 is only 9-9.5" and most cases could house it - have at least an intake and exhaust fans in case though
That depends on your needs. At the moment you are ok with the mATX batuchka suggested. This will leave you with only the PCIe x1 slot available though. The GPU will take up the PCIe x16 and cover the top PCI slot. The wireless N adapter will then fill the last PCI slot. If you need more then that, I would recommend an ATX motherboard.
EDIT: I look away for a couple minutes and end up posting late
RAM draws power through the DIMM slots on the motherboard. The motherboard regulates the voltages. Many times I've had the default setting of AUTO for RAM voltage work fine, but I have run into instability issues that were due to inproper voltage settings for the RAM.