I agree with KF about the rear amps -- they are not nearly as critical as the amp driving your front speakers. It is not necessary to buy a new 5-channel HT amp unless you have a desire for a serious upgrade. If you use your 2-channel amp for the rear, you will (obviously) need a 3-channel amp for the left, center, and right speakers (assuming you're planning a 5.1, rather than 6.1 or 7.1, surround system). This 3-channel amp should be of good quality to drive the Maggies you are considering. Ideally, your center speaker should be a Maggie, since the center channel should have very similar sound quality to the main speakers (this is particularly true with Maggies which have a distinctive sound).
With regard to power, I have found that in most home theater systems, 100 watts per channel is more than enough for the rear channel speakers, unless you running large, full-range rear speakers. The Maggies, however, really need more power than that (I think 200 wpc is about the minimum), particularly if you will be using them for music played at moderately high volume levels. Maggies are a bit difficult to drive, so you should do some research in the Audiogon archives for threads about amps that are best suited to Maggies.
With regard to power, I have found that in most home theater systems, 100 watts per channel is more than enough for the rear channel speakers, unless you running large, full-range rear speakers. The Maggies, however, really need more power than that (I think 200 wpc is about the minimum), particularly if you will be using them for music played at moderately high volume levels. Maggies are a bit difficult to drive, so you should do some research in the Audiogon archives for threads about amps that are best suited to Maggies.