1. I would recommend running a separate VJ laptop, install the RTPMidi on both of them and then hook them up together using an Ethernet cable, then you can tell Ableton to send midi data to the other computer to trigger the clips. What you want too look for in a Laptop is one with a good graphics card, Buy a used laptop if you're looking to save money. My buddy bought his Elitebook for $250 off Craigslist. You should put an SSD in the laptop too for the best performance.
2. DualHead2Go works great, lots of people on here use them. If you're using the DP edition, get the Display Port to DVI adapters and then buy a DVI to HDMI cable from monoprice for the best video quality. Don't use VGA cables.
3. The resolution of your footage should ideally match the native resolution of the projectors that you're using. If you're playing small venues, I highly recommend using an Ultra Short Throw projector as they can be set up like 2-3 feet from the screen. If you're not looking to spend that much on a projector, the Benq MX880UST is good and you can pick them up cheap Refurbished http://www.geeks.com/details.asp?invtid ... -R&cat=CON
. If you got money to burn the NEC NP-U310W is really nice, it's brighter (3100 lumens vs 2500 for the Benq) and it's native resolution is WXGA (1280 x 800) 16:10 widescreen versus 1024 x 768 4:3 aspect ratio for the Benq. I used the Benq for a show and it was plenty bright for the bar we played at.
When you re-encode footage to DXV (Which you should do to get the best performance out of Resolume) in Mpeg Streamclip or Adobe Premiere, You should change the resolution of the footage to match that of the projector you're using for the best performance in Resolume.