Without a doubt, singing better is the quickest and cheapest upgrade, but if you record other people's voices, you should first buy more mics. You need at minimum a small and a large diaphragm dynamic mic and one of each in condenser flavor; after that, upgrade your preamp or perhaps add a high-quality channel strip with compression and EQ. (bonus points if it has both opto and VCA compression) If you record acoustic instruments and have a room that's up to the task, a matched pair of small diaphragm condensers is next. Then, you'll want to investigate ribbon mics and exotic 'character' preamps. The order I'd buy 'em in? SDD: Shure SM-57, LDD: Shure SM7B, SDC/pair: Rode NT5, LDC: Guage ECM-87, first ribbon: Cascade Fathead II. I can sympathize; I got big plasma scars in the crook of each elbow from back before they had the machines. They used a big-bore needle and plastic pouches that only held a pint. I don't sell my swimmers, personally, but hereabouts, it goes for $65 a squirt and up; there's a premium on grad student sperm, (I live in a college town) and certain genetic profiles can get upwards of $100 a pop, or so I'm told.
