GTA4 runs best on a quad. you may want to look into something like this (if your motherboard can handle it, of course):
AMD Athlon II X4 Quad Core 620 2.60GHz (Socket AM3) - Retail [ADX620WFGIBOX] Processors AMD Athlon X2/X4
AMD Athlon II X4 Quad Core 620 2.60GHz (Socket AM3) - Retail [ADX620WFGIBOX] Processors AMD Athlon X2/X4