just copy the important data from your current boot drive, be it folders or files or whatever, to a dedicated section (for example make a folder on the 300GB drive called "Old 80GB C Drive" or something, whatever you want), & copy any important data from the old drive to it... for example you can copy many if not all the folders in the My Documents folder to it, etc etc... then with the new drive, once windows is reinstalled (assuming its the same version as now, if its XP now & you'll reinstall XP with the new drive), then copy the folders back to the My Documents folder, some like My Pictures will ask if you want to over write them, just click yes & the stuff should be there (just remember to copy & paste, not cut & paste, sometimes doing the latter if something screws up, the data "cut" to the clipboard is lost, so can't try a 2nd time)
one method of backing up important data.