oookay, is it a small form factor case?
if it is, the motherboard is probably custom for the case, as in wide enough to reach front an dback so that the front I/O doesnt have to have wires, that typ eof mobo doesnt fit in normal cases, what you can do is buy a new motherboard, just a cheapo, then get say, a sandy bridge cpu like the pentium g840. ( @80$ ) and a cheap h61 mobo, prob around 60$. i imagine your current dual core is a pentium 4, in which case the only things you could upgrade to with that motherboard are core 2 duos and maybe core 2 quads. which the pentium g840 is already better than i think all core 2 duos, and mountains better than a pentium 4.