We do 32 tracks via Reaper off the X32 via USB - no problem. We only run 48/24.
Different people have different ideas of cheap, especially those of us who are hobbyists. I was assuming the OP was a hobbyist. If he was a professional, he probably wouldn't be asking the question on this board. To me an X32 at $2800 is an expensive tool for professionals. An HD24 for $600 is a little expensive, and "cheap" is less than that. Most of the people that I know who record to computers, and I don't honestly know that many, they all seem to be frustrated with dropouts or incompatibility. To me interfaces are for people recording their own band in their basement, where if something happens they can just do another take. You probably have different experiences and opinions. That's fine.
So - why the Firewire? Since no PCs come with it - just buy the right card! (vs buying a whole machine to get one feature!)
Buying a good firewire card for a computer which doesn't have the right chipset on the motherboard will fail too, unfortunately. Been there, done that. I'm just mentioning this because I didn't want the OP to see some cool box on eBay and grab it assuming it would work with any firewire laptop (like I did) and then not have it work. That's all.
The biggest caveat with getting into multitrack is having a the TIME to do the post production - and a PC with enough horsepower for mixing/processing.(vs mastering which isn't as intensive)
Absolutely. Those of you who mix down whole shows, how long is typical for you? For me, it seems to be a week's worth of evenings for a 2 hour show. At that point it's good enough to put it up on the LMA or make demo disks, or hand to some mastering guy who knows what he is doing.