I love microphones (which is probably why I own so many).
If you are thinking of using your microphones strictly for PA purposes only, then you should consider some sort of cardioid or hyper-cardioid microphones. Dynamic microphones do not require external power and tend to be less prone to feedback. Condenser microphones can either be internally (battery) or externally (phantom) powered and are often more prone to feedback. Most folks prefer the sound of a condenser over a dynamic and condensers tend to be more sensitive when compared to dynamics. Shure 57/58s are dynamics and MXL 900 series mics are condensers.
If you are sure you will not use your microphones for anything but live PA use, then you may want to consider getting a set of good dynamic microphones. If you will be using them for live vocal use, make sure that the microphones have a good built-in wind/pop screen. If you will be using 2 microphones for instruments only, you can consider getting a decent pair of condensers. The diaphragm in condenser microphones are generally more delicate when compared to dynamic mic diaphragms. So using them for instruments rather than vocals (sensitive diaphragms hate moisture and wind from your mouth!), is okay and will also give you the ability to occasionally use them for recording.
So my suggestion is to get 2 decent dynamic microphones (for vocal PA use) and 2 decent condenser microphones (for instrumental PA use and occasional recording).
Which mics? I too have been bitten by the MXL bug and own (2) 4000s, (2) 2001s, (2) 603s and a V900 for a more vintage live appearance (band standing around a single mic).
http://g-ecx.images-amazon.com/images/G/01/ciu/43/cb/507de03ae7a04a6449533210.L.jpg
Of those mics, I really like the sound of the MXL 4000s. They are smooth, warm, clear and can be set up as cardioid, figure 8 or omni-directional. They work very well for recording, but may be a bit too big for PA use (but sound amazing in a PA) and watch out for feedback!
Probably your best bet on the cheap is to get 2 Shure SM58s (or a less expensive quality knock off like the ES58 http://www.speakerrepair.com/page/category/microphones.html ) and a pair of MXL condenser instrument microphones like the 603s http://www.amazon.com/MXL-603-PAIR-INSTRUMENT-MICROPHONE-SHOCK-MOUNTS/dp/B004Y36OSQ.
Whatever you decide, have fun choosing and learn how to use your new mics to their best advantage.