First you have to know the types and sizes of files while still leaving room on the primary for folders tucked aways from view. Not only are folders for various softwares found in the Program Files directory but take a look under your user name in the DocumentsandSettings directory and look in the Application Data and other sub folders. Installation logs, uninstall information, etc. are found there. That's how Windows apparently keeps track of things.
The first thing however you want to do is get familiar with a good partitioning tool not just the Windows installer for being able to alter the one or more you decide on. Once you split a drive up too much with several partitions for storing one thing on one and another on another and run out of space on one.. ut oohhh..
For storing files right on the primary as well as on a second storage partition(or here another drive entirely) I create custom folders seeing sub folders for each file type like wavs for assigning sounds or jpgs from screenshots or simply wall paper. If you have that need to be on C in order to be used you have to leave enough room there as well as free space reserved for virtual memory namely the paging file.
That is one reason why seeing a good 80gb or more for the C is generally a good idea to allow some free space there as well as having a good sized separate storage partition. The large storage partition of 350gb to 400gb should work well while still allowing upto 200gb for C. If you later decide to dual boot you can alway shrink some of the free space down to see a second primary or root+swap for Linux there. Meanwhile all of your important files are safe guarded on the large 300+gb storage partition in case something goes sideways in someway.
I'm glad to see you were able to catch a good deal on the better ram by OCZ since newegg only saw the value ram for a slightly higher price. If the 640gb model was available when first putting the current build here together I would likely have a pair of those over the 500gb drives in use if not the 750gb size. That will still work well there. Before making a final decision on just how you want to partition the drive have a look at how the XP, Vista including a second storage partition and two for the root and swap for a Linux distro plus an external drive look in the Disk Management tool.
Along with the savings you saw on memory you can also save on the cost of any 3rd party drive partitioning tool. Even running Vista and seeing some improvements in that area by MS I still end up using a free Linux drive tool the Gnome Partition Editor or better known as GParted. The live for cd versions are only 35mb in size and burn easily onto a then made bootable cd-r.
The first thing to do however is get familiar with the guide for using it as well as the Disk Management tool found in "Control Panel>Administrative Tools>Computer Management>Storage" since you will often be using both at different times. The information on GParted is easy to follow at GPARTED DOCUMENTATION - GENERALITIES
Still waiting to see if they will release a newer "platform independent" release then the 0.3.3.0 version found about half way down the page seen at SourceForge.net: Files
That would be #15 from the top when clicking on the "+" on the left of each release to look across and see "platform independent". The others are not able to create MS type primaries while still being able to resize them. And for burning the free iso image onto a cd-r there's a few free burning programs like BurnOn and Deep Burner available.
The free version of BurnOn has been reliable here if you ignore the IE popup to buy the full version once a burn is completed. BurnOn CD & DVD - Free DVD Burning Software - Freeware CD burner Windows XP & Vista
Once burned to a cd-r you would boot up with that like you would the XP installation disk. The one thing to know however is that all you need to do is press the enter key about 4 times once at each prompt to see the defaults loaded. That will 1024x768 for a screen resolution and don't change the 24bit to 32bit since Linux is 24bit there.
Once at the main gui simply refer to the guide until you try a few practive runs at it. After using it a few times you'll soon think this is ridiculously easy!