You drop an SR20DE in it! Oh... wait... umm... anyway, there are a few parts out there for the base engine, you just have to look for them. for free you could remove the intake resonator (I'm assuming that it's like the SR, maybe it's not, I'd have to look), but then it might be a bit loud...
Basically, in order to get a few more ponies squeaked out you're going to end up making it louder and/or rougher (unless you want to dig in and drop some pullies in it or something), and I'm not sure the wife would like it. Try that dead forum, the Sentra section.
On rims, you could always get a set of 2000+ SE rims, there are 15's, but also the Performance Package 16" 6 spokes look great, IMO, just don't get the OEM Firestone Firehawk tires (if you could find some, which is decently likely, check b15sentra.net). Good luck!
16's-