I'd suggest getting an imaging program (Symantec/Norton Ghost or Acronis True Image). I know there is a free one out there too, but can't think of it off of the top of my head right now - DriveImageXML, or something like that, I think
I believe Western Digital includes a full copy of one of those (Acronis I think) in it's rescue CD/DVD - you could use that too.
Most will allow you to change the partition size, but if not, you can certainly do it once in the OS - Win7 has a nice partition extender built in.
Whose 500 GB unit will you buy? Most HDD makers have a utility for this purpose you can download for FREE from their websites. They are customized and will only make a clone copy TO their own drives. So if you buy WD, get their Acronis True Image WD Edition. If you buy Seagate, get their Disk Wizard. Etc.
What you want to do is called cloning - making a COMPLETE copy of everything from one disk to another in such a way that the new HDD is bootable and ready to take over the role of your C: drive. Most people doing this, like you, are migrating to a larger unit. so watch for one detail as you run the cloning software. By default many of them will offer to create on your new empty drive a Partition on which to make your clone that is the SAME size as the original. But you probably want to make it much bigger to use the entire capacity of the new HDD in one logical "drive". So as you start setting parameters, find the way to manually set the Partition size the way you want, don't just go with the default settings. That way you don't have to re-adjust the Partition after it's all done.