So, I've seen my fair share of 64-bit threads with people not knowing which one to choose and people saying it doesn't make a different etc etc. So, I did a test. Read on for the results. (All results in seconds) Adobe Photoshop CS4: 64-Bit: 2.9 32-Bit: 8.8 Microsoft Office Word 2008: 64-Bit: 7.1 32-Bit: 7.1 Mail: 64-Bit: 1.4 32-Bit: 1.7 iMovie 09: 64-Bit: 7.2 32-Bit: 7.7 Safari 4: 64-Bit: 1.4 32-Bit: 1.4 All results are averages. Each application was opened 3 times on each respective kernel. Results may vary. Testing conducted on June 2009 MBP, 2.8Ghz, 9600M GT, 500GB 5400RPM, 4GB RAM. I think everyone should use the 64-Bit kernel. It's a bit faster, and hey, you payed for it!