Here's an important rule of computing, especially for someone like Mamba who is obviously an 'intermediate' level user: Only install and configure what you need as you need it.
More specifically, unless you have already found a reason why you would need to run XP, as opposed to just emulating it with a VMWare installation, setting up a dual boot, especially using an OS that is now 11 years old on a system that is brand new, is the epitome of stupid. My work laptop has no end of problems because our IT department actively wipes new machines as they come in with Win7 pre-installed, and then sticks an XP image on them. We have finally convinced them to build a Win7 image after reporting crashes, lockups, and random system errors for over 12 months.
Home premium versus Ultimate. Again, it's what you actively need. I have Home Premium on my laptop, and Professional on my desktop, and not only have I never really even consciously noted the differences (though I do use some of the Professional features that Home Premium lacks), I've definitely never had any need to have ultimate installed. Here's MS' comparison chart:
http://windows.microsoft.com/en-us/windows7/products/compare Unless you have a need to switch language packs on the fly (I doubt), there is no reason to use Ultimate. BitLocker is junk compared to the excellent open-source TrueCrypt - which can encrypt your entire drive. Professional does have an XP Mode that is more seamless than VMWare, but despite the fact that I have it installed on my desktop I have never needed to use it. Most of the features that Bob-san mentioned are actually available in Professional Edition, so you don't even need Ultimate there. As for the RAM cap, this is a laptop, the chances of you significantly upgrading the RAM are nil considering by the time you actually need more than 16 GB of RAM you're probably going to want to just replace the system. Laptops have a typical replacement cycle of 18 months to 3 years. Much like smartphones and tablets, laptops are now considered throw-away technology.
Now, if you MUST have Professional (and I'd say that's a dubious claim at best, given what you've demonstrated you know about and do on a computer thus far), it's $100 retail to buy, or as The E said, check if your school is part of the MSDNAA program and get it for basically free (and legally).
Under no circumstances would I put any Windows OS other than a 7 edition on a machine built in the last 1-2 years. It's unnecessary and just smacks of uninformed obstinance, particularly if YOU. HAVE. A. LEGAL. WINDOWS. 7. LICENSE.
(Oh, and if your machine did not come with a Windows 7 disc and you want a clean install without the bloatware, you can use any Windows 7 DVD to install it and then input your license key - all Windows 7 discs come with all editions on them, it's just the license key that unlocks the particular features you use.)