A lot of older mobos have 4GB limit. Even just a year old, some have 4GB limit. It's more common than 8GB.
The ASUS brands are normally good, yes, but the features and capabilities of any given mobo depends more on the layout, the design, and how badly they wanted to save money (if it's a budget board, which this seems to be). Anyways, it's got 2x DDR1 slots and 2x DDR2 slots. You can only use 2 at a time (no mixing of memory types allowed).
My current "upgrader" mobo has a similar feature.
A 256MB Ge 7600GTS/GT runs around $120 nowadays for PCIe, so you don't have to spend $300 if ya dun' wanna

The system you described isn't bad. Like you said, not the latest but still capable of playing AH. Not sure how much upgradability the socket 939 has (aren't they doing away with that? Or am I thinking of something else?).
For me, it's Socket775 all the way

(but don't let that influence
your choice, dicho!)