completely new to home Linux

Hi all,
I have experience with Unix and Linux (red hat) at work, but would like to install a dual-boot option on my new home system. I have a disk copy of Ubuntu, i'm just now formatting my new hard drive.

I don't know *squat* about this; completely learning as I go along.

What I want-
a system capable of doing everything I do at home with XP Pro, yet with the freedom and flexibility of Linux. I'm not much of a MS fan, to tell the truth.

I need all the usual; web access through my wireless G and/or ethernet card/cablemodem, SSH tunnel to work and back, Star Office or openoffice complete utility. I regularly run fairly heavy numbercrunching programs and 3D modeling programs, my new system's running an AMD 64, 250 GB Seagate HD, within a few months will be running raid 0 on another 250 GB hd.

questions:
how much partition space do I allocate to each OS?
How to I prompt the system to invite me to select which OS i want to use? I'm not the only user on the home box, but i am the only one who'll be using Linux.

any other advice etc. is totally welcome.

For the record, i'm more hardware savvy than software savvy, which is probably obvious from my questions; as a mechanical engineer, I generally just let software 'do its thing' and tweak my hardware, but I am interested in this move to Linux.

thanks
k wallace
.