Short answer, yes you can run a 64bit kernel in a 32bit userland, but there will be occasionaly oddities... tried running top yet? Check out this post: http://www.uwsg.iu.edu/hypermail/lin...06.0/1725.html
for a reason not to... You will want the nvidia package that matches your kernel... It's not a problem to go on as you are, I only include that post and the warning because I see no reason not to switch to pure 64 bit, if theres something you cant find ported to 64 bits, you can install 32bit libs in a number of ways....
Debian runs pretty good on my AMD64 X2 3800 too