Ext vs. Int HD's

OK -- now i know it can be sorta like comparing apples and oranges, but ==>

I'm considering getting an additional HD.

in my tower I have an 80gig
I also have an Ext 200 gig [which fills up very quickly -- it is used strictly for backups]

If i were to get an additional external -- i'd be looking at around a 500g. Buy.com has a Seagate for $130
or i could go internal and use it for big files [many photos and kids stuff] plus for backups ... newegg.com has a 750gb WD Caviar SATA for $200 [or even a 500g for $110]

now -- question... say my PC crashes [which in 10yrs has never really happened] will i be able to access the data on the additional internal drive [considering that is where my backups may be] ... or i wouldn't be able to access anything since it wont boot?? [which is logical]

I guess I'm trying to figure what the better option is... or just get BOTH and use the larger EXT for my backups and the larger internal for larger files. I have 5.3gb of images and my son has 5.5gb of 'stuff' [mostly Stepmania songs].

any input would be great.

TIA,

Terese