if you want a Mobo, Memory, AND a processor, $350 isn't going to get you much. And whether or not you'll see any increase over the specs you've quoted is even more questionable. I don't know how much stuff goes for in $, but for the £250 at best that that's equivalent to, you'd end up skimping on one of the three to stay in budget.
e.g maybe get a 3700+, some decent mobo lol, n then you'd have little left for RAM.
Well, that's how it seems to me.
Also, you might wanna say what you use the pc for aswell, if you encode lots and don't game, it'll make a difference to people's suggestions.
Good luck,