How to upgrade my computers - both Sony Desktops?

i have two computers, both are Sony desktops. I am considering upgrading them but i would like to know how hard this is to do. The reason for upgrade is that I've used allmost all of the internal disk space on the hard drives. I do a lot of photography and it's eat up the storage. Is upgrading / adding a harddrive an easy task, or is it more complicated? Maybe on a scale of one to ten? Is this something that I can do myself? I'd like to keep the old harddrive and add a new one, not replace one for the other. Also, do I need to upgrade or add RAM when i add a harddrive? are there any conflicts/issues with one being newer or larger than the other? is there a website(s) that will tell me this sort of information? Or should I just go to a professional? The problem is I wanted to do this for Christmas present, and the only price quote I can get is almost $200 for installation and approx. $100 for the disk, and that was a little more than I wanted to go. Thank you.

How to upgrade my computers - both Sony Desktops?vista

It's a 2 on a 10 scale if you know how to follow directions and you probably do! Go to bestbuy.com and navigate to the upgrade section. It should be able to tell you what you need. You'll need RAM, Harddrive and that should be it. Maybe a Bios update too which you can get from the Manufactor. Total cost: $300 for both