Best way to check the compatibility of RAM is to go to the mfg website, enter the mfg/model of the mobo, and they'll tell you what is compatible. I would go with the DDR2 800 given it will cost you about the same
The antec p150 is one of my favorite cases. It is being discontinued, being replaced by the sonata designer 500. They are both good cases, and are particularly quiet. Both include good power supplies, which you already have. The antec solo which g-paw recommended above is the same case, in black without the included power supply, and is priced accordingly lower.
The newly announced E6750 2.66ghz is a faster processor, and costs less $205. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Looks like a nice system and should be very fast the only thing I might consider changing is the HDD, 500gb is the sweet spot right now and you could get one for about $20 more but other than that your system looks great.