I've done the breadboard stuff before, and used handyboards for robotic development at school. My only problem with these is that they weren't very powerful and didn't interface with anything else well.
I don't really know of anything aside from USB/Firewire that will provide modular components like I'd like. I also suspect that if I wanted to go down that road it'd be even more expensive. Thats one advantage of using something more common in everyday use like USB that it will be cheaper.
Feanor: $1000 on just peripherals seems excessive.
On wifi: being able to quickly switch access points or what not isn't a huge deal. I mainly plan to use it for information relay/debugging to avoid cables. It will probably also be used for remote control in the start.
So heres my list of goals for now atleast:
1. Remote control drivable, w/video
2. Autonomous with simple sensors, and simple tasks like avoid light or just move around avoiding obstacles, etc.
3. Autonomous and traveling along GPS waypoints, the idea being that I can give it GPS waypoints down my street and it will be able to travel there based only on its knowledge.
Edit:
I came across
http://www.epiacenter.com/modules.php?name=Sections&sop=viewarticle&artid=61 which talks briefly about making a PC based robot. I also found
http://www.linuxrobots.org/ which seems helpful.