Well you can
try to change your latency for the MIDI (should be able to do this through whatever software you are using, and some MIDI interfaces can do this as well), and you
may be able to change the latency for the DVforge as well, but i have had no experience with this. Although it is worth a shot, it is a long one.
You asked for
any idea, so here one goes. Record the drums first, then playback the drums and record the guitar and keyboard to what the performers are hearing. If you record the drums with a click track, this should work fairly well, but i know that this defeats the purpose of the group recording together.
If all else fails, record the drums first, then record the keys, then record the guitar. If you go this route, i would recommend to run all of the audio through the mixer, just one instrument at a time. That way there would not be a notable difference b/t the drums (recorded via mixer, then line in) and the keys/guitar (recorded via usb). This may be your overall, best bet, as usb transfer speeds will fluctuate depending on processor load. This is the reason why most people prefer to record audio onto a computer via firewire, spdif, or lightpipe.
That said, if you want the cheapest breakout box, look at the
M-Audio Firewire Audiophile. It has MIDI i/o, two analog inputs (rca), and four outputs (also rca). At $199, it is the cheapest "right" way to do it. So for instance with this box run the keys via MIDI, the drum mix via one rca, and the line from the guitar into the other rca.
Back to your original post my Ti PB 1Ghz, with 512mb ram handles everything i throw at it, so i would not think that it is a ram issue, although the more ram the better (i think i am finally going to break down and upgrade to 768mb, ram is still too expensive for my tastes.)
-cameron