I'd probably nab a quad-core Llano for around $100-110 and have at it. Pair it with a a55/a75 chipset FM1 board and 4GB or 8GB of DDR3-1600 and you'd be set. Impressive onboard graphics, solid (not ground-breaking) CPU performance and a great value. You could easily do a rig for under $600, with a SSD and OS.