For 300$ you could pick a Gigabyte DS3, a C2D E4200 and some DDR2. If you manage to sell your x2 3800 and the DDR you should get enough to get a 2x1GB DDR2 kit.
At this point sk 939 is a dead end, AM2 is pointless and here's why:
- sk939 is discontinued, so if you decide to buy a mobo for 939 to replace your old one you'd be better with a cheap mobo since the money you're investing will never be recoverable.
- in q3 (around September) AMD would launch AM2+ and later AM3. The CPUs based on AM2+ are compatible with the AM2 mobos, but they would not run at full speed.
A DS3 is around 130$ and the E4200 should show up very soon at a price around 130-150$. A review on it here
, it OCs over 50% on stock cooler...