A lot. I've had music playing while web browsing, checking 5 or so forum apps, with email updating every 5 minutes and Facebook every 30, and a few other random apps and never had a problem with any of them closing on me and causing little to no noticeable lag. I'm overclocked to 1.2ghz at the most and that's not much higher than stock, and this phone has handled everything I've thrown at it with no problem. I'm not really a super power user though so if you are it may be different.
