Ok, another shameless plug:
Get my pair of ADK SC-T mics:
http://taperssection.com/index.php?topic=131347.0Pair those with a cheap phantom power box if you really want to save money. Or best yet, get a littlebox preamp from naiant.com.
Shameless plug aside, I think a pair of lower cost condensor mics like my ADK's, the Studio Projects C4's, or the Avantone CK1 mics paired with a littlebox preamp is a great answer to your quest. All of these mics come with at least cardioid and omni mic capsules which provide a lot of flexibility in recording, and all sound very good for the price. The littlebox preamp which starts at just over $200 provides phantom power for condensors and sound very good. Both myself and Chuck here at ts.com have run comps of the littlebox against expensive preamps like the Grace Lunatec V3 and EAA PSP-2, and the littlebox has faired very well, with more people preferring it in blind comps.
A littlebox preamp and a pair of any one of the mics I've listed should put you in your $500-600 range, and should be a noticeable step up from the m10 internals.