Smittefar
Axe-Master
^^^^This! Most excellent advice! I went with a pair of these: http://www.sweetwater.com/store/detail/Opal
Yeah, I know, really pricey but in this case you DO get what you pay for and these were a superlative purchase!
Agreed I see people here advocating that you buy monitors with big speakers, i.e. 8" - I agree that they give a nice oompf, but when on a budget, I would prefer linearity/quality over low-end oompf. As long as the speakers can play 80 Hz, it should be OK for (six string) guitar) - then you can always add a 'cheap' sub if you need a bit of Ooopf for other things.