Yes I know nvidia is terrible for mining, but its not so bad if you have free electricity , or need heat, etc. And there is tons of posts about people just mining on off time with nvidia sooooooo
For those of you with Nvidia this is what I did to get a 450gts to 48 mhash, constant, with <1% cpu usage (other miners where much slower and used 100% of 1 cpu core)http://forum.bitcoin.org/?topic=3878.0
Get gui Miner above, install, click file>new miner>CUDA
Obviously fill in your pool details.
for flags use -cuda and you want to also add -gpugrid (make sure this is number of cores on your card) and - gputhread (will need to mess with this, but don't use unless speeds suck)https://en.bitcoin.it/wiki/Mining_hardware_comparison
Link above this shows cores and speeds for many card, most all nvidia are wrong so make sure to know your number of cores.
For GTS450 I used this for flags
-cuda -gpugrid 192
-gputhread changes the hash rate when you mess around with it, but I think the Cuda miner will try to pick the fastest setting when starting up, so make sure to not have crap running like virus scans or moving your mouse, playing games or watching videos, because it may pick a shitty setting.
Just sit there when you click start mining and wait to initialize.
I also Oced mah card to 945 clock using, ran at 52c no problemohttp://event.msi.com/vga/afterburner/download.htm
If this helped you please send some BTC mah way, if I get some I might even post some more detailed help!