LiFePO4 cells are the way to go!

They have many of the advantages of LiPos but a much lower output voltage (2-cells come to about 6.2V after they've settled off the charger).

There are a growing number of different cell-sizes now available and they're *dirt* cheap ([link=http://www.dealextreme.com/details.dx/sku.1880]an example[/link]).

I've been using the 1350mAH cells in my .40-.90-sized planes and will be using those 350mAH cells in my 1/2As. At $4.50 per pack/2-cells (plus lead and heatshrink) they are pretty cheap and can be fast-charged in under 30 minutes from flat.

There's more on the difference in the battery technologies [link=http://www.rcmodelreviews.com/baffledbybatteries.shtml]here[/link]