There is a nice modified version of aircrack-ng I found on their website aircrack-ng.org. It is optimized for CPUs with the SSE2 instruction set (what is SSE2?). For classic dictionary attack on WPA key, it is able to speed things up to 500% compared a not optimized version. I made a test using BackTrack 3 with CPU Intel Core2Duo P8400 2.26 GHz, cowpatty 4.3, aircrack-ng 1.0 rc1 and aircrack-ng-wpa-sse2. For this test I will use a dictionary and a handshake (wpapsk-linksys.dump) from Cowpatty package. The results are on the screenshots.