The E6600 does seem worth it for only 50 more, people seem to be universally for it. tigerdirect has it for 249.99
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2341562&Sku=CP1-DUO-E6600
On the note of dedicated sound card, it seems cheaper to invest in a mobo that has 5.1/7.1 built in, as long as it's 5.1 or higher I wouldn't be looking to upgrade. Been looking at motherboards for about an hour now, talk about in over my head! But as stated earlier, going for 2GB RAM, it's pretty easy to find it on sale for a great price if I actually put forth the effort to find it.
While I was on tigerdirect I noticed 8600 SLI compatible cards are in the 160 range, which isn't really that bad. I don't have any experience with nvidia cards after 7800, so not really sure what i'd be getting into performance wise. But if it's SLI, I can always double up later, just need to get the right psu right from the get-go.
It seems the more I look, the more confusing this gets!