Before going down the route of spending lots of cash on interface, new PC, sound card, preamp etc. I would concentrate on getting a good studio mic. This will probably resolve most of the issues and at least give you a ref point for further improvement. The recording chain is only as good as its weakest link and with a crappy mic as a starting point no amount of editing or preamps etc will fix the problem. There are some pretty good condensers at around $100 and some pro mics can cost thousands....I suggest looking on ebay for a good second hand valve or tube studio mic.

http://shop.ebay.com/i.html?LH_Site...one&_fln=1&_trksid=p3286.c0.m283

Something like AKG, CAD, Studio projects, SE, Rode, or Bluebottle (great mics if you can afford one)
I would stay away from Samson and Beheringer and similar cheaper makes.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055