Now that we've pared this query down to one post, I'll answer it.

Before you go LiPoly, be sure you understand all the issues involved. We wouldn't want you burning down your house because you didn't follow the special handling and charging rules... There are many threads on proper handling of LiPoly cells, and most battery vendors have specific instructions and warnings right on their websites. If a vendor does not have that information available, find a different vendor.

The conversion method requires you have some basic electronics knowledge. You need to know about Volts and Amps, about series and parallel, and about the concept of "C."

C is the Capacity of the cell you're using. For example, a 900mAh cell has a C value of 900mA, or .9 Amps. A 1200mAh cell has a C of 1200mA or 1.2 Amps. You use C to determine charge rates on all types of batteries, and discharge rates on LiPolys. When you go shopping for LiPolys, they will have a discharge rating in terms of C. This tells you how many Amps you can draw from the cell without damaging it.

You need to know how many Amps your T-Hawk motor is drawing to choose the right LiPoly cell. You can measure this with an ammeter, or research the value. FWIW, a typical 380 motor setup can draw up to 12 Amps. This means you need a LiPoly that can handle a 12 Amp discharge rate. Shopping around, you come across a Kokam 1500 LiPoly that has a capacity of 1500mAh and a rating of 8C. 1500mAh at 8C gives you 12 Amps of discharge capacity. If you needed more, you can gang multiple packs in parallel.

Voltage wise, it's Volts for Volts, or as close as you can get. The 7-cell THawk battery has 8.4V. A 2-cell LiPoly pack has 7.4V, and a 3-cell has 11.1V. The closest you can come is with a 2-cell pack. One Volt won't make that much difference, especially since the LiPoly pack is lighter, and won't drop voltage as drastically under load as a NiMH.