Yo, Theg! Welcome to the board! There is a cool "search" function that could have already answered your question, because this board has been debating that question for *years*.
Anyway, here's my version of the short form. Marshall MXL603 is the wicked cheap winner, a fair compromise between the needs of drum overheads and acoustic guitar. For piano, I prefer large diaphragm mics, but that's just a personal preference. Oktava MC012 is also a good choice. Quality control has been a problem, so you want to hear the mics before you buy them. That generally goes for all mics, in a perfect world. The Oktavas are more versatile in some ways than the 603's,with pads and the availability of omni and hypercardioid capsules. In short, I think they rock as drum overheads but are mediocre on guitar.
Studio Projects C-4's are closer to $400 for the pair. They are very good on acoustic and really clean as overheads, They'll give you a very sharp image, but I would not choose them for something that should be a little muted, like vintage blues. Rode NT5 are in the same ballpark, but those mics I've never heard. I've heard no one say they totally sucked, and that, itself, is probably indication that they're fairly good mics. Both Shure and AKG are starting to make forays into the budget small diaphragm condenser market, but the mics in question haven't been out long enough for the recording community to come in with a verdict. The 603's and 012's are cheap, and work very well, and the NT5's and C-4's seem to be fairly priced for what they do. Personally, I use the C-4's for guitar and instruments, and for Country or Jazz overheads. I use the Oktavas or AKG C2000B's for overheads. The 2000B's are mid sizad diaphragms, and actually work pretty well for overheads, guitar, vocals, and toms, and are in your price range. They are particularly good on hard rock, like early Ted Nugent or Deep Purple. Anyway, that's my story, and I'm stickin' to it.-YMMV.-Richie