I would change out the bulk of your build, because you can do considerably better for the same amount of money and even cheaper. Here's a list of components for a build I made up a few days ago, from Newegg. It should give you a rough idea of what you can get, and use it as a base. In my opinion the best upgrade you could achieve if you wanted to spend just a little more (~$10-25) would be to go with a slightly better Llano. This would give you a slightly more powerful CPU and GPU. Depending on your setup and what you do regarding your HDD, you could go all the way up to a nice quad-core Llano. Just something to think about.
Motherboard - ASRock A55M-HVS FM1 (Hudson D2) Micro ATX (59)
CPU - AMD A4-3300 Llano 2.5GHz Radeon HD 6410D (65)
Memory - G.SKILL Ripjaws Series 4GB DDR3 1333 (PC3 10666) (22)
HDD - Samsung Spinpoint F3 500GB 7200 RPM 16MB (90)
Optical - LG 22X Super-Multi DVD Burner Black (16)
PSU - Antec EarthWatts Green EA-380D 380W (45)
Case - HEC 6K28BB8F Black MicroATX Mini Tower (35)
The total of this list comes out to $322 before combo deals, MIRs, and shipping. You can cut more money off if you can find a cheaper HDD somewhere online, and/or a smaller one. The case also is something that is very dependent on the person.