I highly recommend that you not take anything personally and continue to ask questions before you start putting too much work into the set-up. We've all had ideas that nobody agrees and the group is usually saving someone if they listen.
Any PC that can play steam games will be great for mame and I believe you can use attract mode as a front end for both. No need to separate a thing if you really want those on the same monitor. I recommend setting things up so that you play arcade games on a 4:3 monitor and steam games on a big 16:9 HD monitor. Steam games that are gamepad compatible are especially enjoyable on a big screen television while sitting on the couch.
Thanks monkeybomb. Nothing is taken personally. I'm a complete newbie with this stuff, so I expect to ask questions that many consider ridiculous.
To answer the common question of "why use the Pi", the main reason right now is I really like the idea of the Pi, and I don't necessarily have a spare PC that can properly run, Star Wars Battlefront II, for instance. I would likely pick up a Pi since they are cheap, then add the PC later once I have one available.
I guess if I'm going to spend the time building and wiring the cabinet, I should do it right the first time and build in with the assumption a PC is going in. I could always do a mini-cabinet with a Pi as a test run to see if I want to spend the time and money on building a full size unit.
Thanks for the input everyone!!