I have a projector set-up identical to yours in the basement
of my house (Panasonice AE700). Like you have have pretty good
light control (one egress window, covered, or soon to be with
a dark curtain). The room itself is rather large (28 x 23).
My viewing habits are pretty much the same as yours and I, too,
have to get HDTV off of a satellite.
After playing around with the projector for a couple of weeks,
I decided on the 52 x 92 Day-Lite Perm Wall (fixed mount) screen
which, I believe, makes my diagonal something along the lines of
110".
I went with the grey and have been very happy with the results.
Although the blacks from the LCD projector probably wouldn't satisfy
the most dyed-in-the-wool videophiles they strike me as pretty darned
good on the grey screen.
Whatever you do, though, I'd recommend going the HD screen route.
While there's not alot of HD programming available yet, once it
becomes more widely available, you're gonna want a screen with the proper
aspect ratio. (IMO, the AE700 is simply frickin' incredible for HD
programs).