VSTis/software-samplers usually take up a lot of RAM. I barely got by with 512MB with VSTi's on a few tracks. (EZDrummer alone took up about 300MB of RAM). Depending on your MIDI instruments, I would say at LEAST 1 GB. 2 GB if you can afford it.
Hard drive - Go as big as you can afford to. At least 7200 RPM. I use an 80GB but that's because I'm broke. I would recommend at least 160 just for recordings, but it really depends on how much you record.
It's common for people to have 2 hard drives, 1 just to record onto, and the other for Windows and all other software.
Not familiar with the MOTU, but if that's an interface it should act as your soundcard. AFAIK, you can't have 2 sound devices enabled at a time in Windows (I've tried).