A couple of years ago I bought my first digital SLR, the Olympus e420.  It takes great pictures when there is a lot of light, but I'm not happy with its performance in low light.  It seems that if I use an ISO of 800 or higher, there is a lot of digital noise.  I find this extremely frustrating.  I like to take candids and and I don't want to be surprising people with flashes all the time.  I suspect my issues stem from the fact that the Olympus uses the 4/3rds system. Please correct me if I'm wrong. It probably doesn't help that none of my current lenses have an f stop higer than 3.5, either.

So, I want to switch to Nikon or Canon...spending about $800 or so.  Any recommendations?  Or should I just buy a nicer lens for my Olympus?