AMD A6-3650 APU - $84.98
ASUS F1A55-M LX Plus R2.0 - $65.00
AMD 2x4GB DDR3 1600Mhz RAM - $34.99
Corsair CX430 Powersupply - $44.99
ADATA 120GB SSD - $94.99
Total: $324.95
You'll have to use your existing enclosure, and also buy another, higher capacity harddrive to put all your downloads, as the SSD won't be big enough.
On the plus side, you'll have a nice snappy system with that SSD!