If you have $400 to spend on this you are way better off buying a whole new rig. You should be able to get a cheap mobo + CPU + 4GB DDR2-800 ram for around $300. Toss in a new 4850 for $100 and your set. Sell your current stuff for $100+ and either pocket the case or get the new PSU you want. You'll have a way better system and all within your budget.
thats an even better plan