Mac Mini :: Using A Mini SD Card As A Scratch Disk?

I am getting an i7 iMac in the near future (ordered yesterday, won't arrive til Jan. 3rd ) And I am getting it stock with just the 4gigs of ram, and don't plan on upgrading anytime soon, but I probably will at some point when prices come down. Anyway, I use photoshop some (not a power user or anything but I use it for some stuff) and I was wondering if you could use a mini sd card as a scratch disk, and if you can, is it a good idea? Thoughts/opinions/comments.

On to my HD question: should I pay apple to upgrade my HD to the 2TB drive? I think I will definitely fill up the 1TB so I'd like the extra space, but would prefer to buy the drive from ifixit or owc or something to save some cash, but the thought of having to remove the huge glass screen with suction cups and disconnecion the LCD seems a little daunting so I was wondering if I should just save myself the hassle and get the upgrade done by mac (it'd be $225 instead of $250 because I get the student discount) Of course I'd lose out on the 1TB HD that I could later stick in to an external HD enclosure, so its a pretty big hit in terms of cash.

From watching the teardown on ifixit changing he HD looks like it could be a little tricky, but I'd like some input from you guys. I am pretty tech savy (former PC user so I've replaced a lot of components like ram, video cards, HDs, ec cetera) but getting past the wall of glass is daunting, but I may just be hyping it up, so I wanted some input.

Third and finally, I got the wired keyboard because I wanted a # pad but I just found out my old Logitech G13 has mac drivers so I was thinking of switching to the wireless one just so I could have a wireless mouse + keyboard for when I want to watch a movie or something from further away, though ideally I think I'd prefer a keyboard with a built in track pad or track ball, but I don't really want to fork out the cash for it now. So is there any advantage of the wired numeric apple keyboard compared to the Logitech one? I can still control media, I have a remote, and it can do expose and screens and all that good stuff. But as always I wanted to make sure I wasn't over looking anything.