I wouldn't bother buying a 4870 if you just want to get a 4890 after the new cards come out. If you're planning on buying at the end of September, then just wait for the new cards, then make your decision about graphics card. That motherboard is an AM2+ board, so you're right to go with the DDR2. I'll second the recommendation for the Corsair PSU.
I don't think I'm going to go crossfire, it just doesn't seem worth it to me.
Good suggestion on the PSU, it was on sale only for this weekend, so I've already bought one.
I read the reviews on that seagate. I'm not too sure if it's very reliable, but it looks like a pretty good drive.
A friend suggested I go with the X4 945 instead of X3 720...is the performance gain worthwhile?
Its an extra 48 for the fourth core, unless you know you are using programs that can make use of the fourth core it isnt really worth it. The 720 is also a black edition so you can easily OC it past the level of the 945.