Two of the same thread??
What's your ram configuration? If you're on 2x512 consider selling it for $50-60, selling the 6800gt (likely for at least $180-200, more if the buyer wants it because it's rarer now), giving you at least $230.
With that you can buy 2gb of value ram (faster timed ram is unnecessary unless you want to seriously overclock everything) and most of a 939 motherboard.
Let's say you went with that:
$600+$230=$830 budget.
There are many options here!
#1:
Asus SLI mb - $120
X1900XT - $520
2gb ram - $165
total: $805
#2:
7800gt/mb combo: $359
2gb ram: $165
X2 3800+: $313
total: $837
#3:
msi neo4-f: $85
7800gtx256: $450
2gb ram: $165
total:$700
shall I continue?