I have my desktop back but I need to reconfigure the whole thing.

My Windows partitions are full (computer was loaned!) and my Linux operating systems were not updated.

I was going to re-install and have a new setup.

The plan is to have larger partitions anyway and use VirtualBox on the Linux OS/partition.

I'm hoping to obtain some suggestions and advice on how to proceed.

It's not crucial which distro you prefer but that you don't mind suggesting an approach!

It might be convenient if you also use Windows though even if rarely so you might have some interest in this topic!

I have two hard drives: 320GB and 500GB. I was going to use the 500GB for XP Pro since this drive is a bit older but both are SATA.

The 320GB drive is what I was using, though. I'm open to suggestions, however. I have Windows on the 320GB HDD but the Windows partitions are around 25GB each and full. I was going to resize all the partitions including the Linux ones (they are about 5 of 20GB each). There's about 100GB of unallocated space.

I thought the idea of booting Linux on one drive is good and Windows on the other drive. I would have Windows on both drives just for BIOS updates or whatever but the 'Linux' drive would have the smaller Windows partition (mostly unused). Or if there's a better idea, please share!

I am not sure how to set this up, though, as I've never done it. All my configurations have been having Linux and Windows on the same drive. I read that Grub2 can easily configure two drives for booting but how?

I read this in the forum after a search:
f you use multiple drives and switch them in the BIOS, you don't have to worry about sharing drives.

You really want an OS to have its own disk. If you share two OS's on one disk, one of them will end up on the inner tracks and will run much more slowly.

Instead of switching in the BIOS, you can put each OS on its own drive and use GRUB to choose. If you are very clever, you can add a small script to each OS that modifies the default GRUB setting, so you can make a "shutdown and reboot as the other OS" button if you want.
I'm still confused about having both drives having a bootable option. My 500GB drive is viewable but I can't boot it unless I disconnect the 320GB drive.

The 500GB drive is a previously configured system with Windows and Linux but since it's been untouched for a while, I figure I'd check to see if there's any data I want and then dedicate it to either Windows or Linux.

Can anyone advise for this project? I was going to dual boot on the one drive and then move the other drive to another build but for now, I thought I would have one computer boot the two drives (I need a cpu before I complete the other build).

Thanks in advance for any suggestions/advice. I hope having this setup is better than multi-booting on one drive: I find it a bit irritating when you want to resize partitions and it seems logical to have Windows on its own drive and vice versa for Linux (dedicated drive for Linux). Right????