How to upgrade such a PC having only $500.?

Update: 2011build:
Amd Atlhon II x4 635, box cooling
GA880-D2H with HD Radeon 4250
2x 2GB DDR3 1666mHz
18,5" TN D-Sub only
500GB 7200 RPM 16MB Cache SATA 3.0Gb/s 3.5"
355W PSU
Micro-Atx Case
Logitech wireless K350+M280
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