FX1800, you can pick them up on Ebay for cheap, that should work fine with your system provided you are running SolidWorks 2012. I have both 2012 and 2014 on my PC and the newer version is a bigger resource hog when you get into more than a 4 layer layout.
SolidWorks isn't so much dependent on your GPU, it is CPU bound so a higher clock rate and more cores work wonders on big layouts. Dropping your money on a GPU over $100 wont net you a huge increase in performance over dropping $40-$80 on a Quad CPU with a high clock.
FX1800 ~$10-$40 "Will need to pass the compatibility test"
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR12.TRC2.A0.H0.Xfx1800&_nkw=fx1800&_sacat=0
Q6600 ~$40
http://www.ebay.com/sch/i.html?_odkw=fx1800&_osacat=0&_from=R40&_from=R40&_trksid=p2045573.m570.l1313.TR11.TRC1.A0.H0.Xq6600&_nkw=q6600&_sacat=0
By the way you can pick up a older workstation of ebay for around the $400 mark that will handle even the newer Solid works nice and fluid should you want to spend the money later.