It depends on the phone, really. If the manufacturer doesn't make an update, there's a good chance that someone else has done so. CM7 runs on tons of phones, as does MIUI. A good place to look is forum.xda-developers.com - if you find your phone, then there will probably be a custom ROM running a newer version of Android. Typical installation process is to root (as you've already done), then install ClockWork Recovery, download a .zip of the ROM you want, put it on your MicroSD card, boot into ClockWork Recovery and choose install update from zip (where you would choose the .zip you just put on the SD card). Get ClockWork Recovery installed from ROM Manager (in the market, or sideload it from your SD card if you can't find it), then you can just open ROM Manager and boot to recovery if you're not sure how to do so manually.