Thank you all for your input!
I have no doubt that MegaDrum beats them all - was just curious by how much as I have a bit of (disappointing) experience with the others. USB is definitely faster than MIDI. Interesting to hear about the TD-30! But I guess it comes down to the basics - it is busy generating the sounds that you're actually trying to replace (if you're using external VSTs). A trigger has no business generating sounds - its job is to send notes, and as quickly as possible - which is why I love the MegaDrum concept (just ordered one, and can't wait to start playing with it). On the VST issue, I recommend SaviHost - it is a free wrapper that lets you run VSTs as if they were stand-alone EXEs, no need to have a full DAW eating up system resources...
http://www.hermannseib.com/english/savihost.htmDefinitely Windows8 is faster than its predecessors, and 64bit runs circles around 32 (I use that configuration on my desktop, and it runs Pro Tools beautifully - love it when the Mac guys diss PCs even as they're busy rebooting after yet another crashed session). Unfortunately, until I get a new laptop, I'm stuck on Win7 at 32bit (though I turned off all fancy graphics & desktop effects). My current sound card is an Audigy2 (PCMCIA), which runs all the way up to 24/192... and oddly enough I
did notice that higher sampling rates actually tend to get lower latency, to a point (which is the opposite of what I expected, considering it is more samples to process for each second of audio). I will give it a go with that configuration, and if it doesn't cut it I have a couple of outboard USB and Firewire sound processors I can try as well (though I'm skeptical that the interface - Firewire
400 via a PCMCIA Firewire card - might eat up any advantages the better sound processor might give me... guess you have to try to find out!) All else fails, ASUS has some nice laptops for around $500.
MegaDrum ARM x56, Addictive Drums via SaviHost VST wrapper, Alesis DM6 USB kit, Win7 32bit, Audigy2 ZS, 2GB RAM, Vic Firth 3ANs