installing it is easy. push the locking lever out of the way, slide in the drive, and plug the two loose cords into the places that fit. snap the lever back down, and you're done.
as for what are good drives... right now it seems pretty much all of them are good, but most of them have had problems in the past. My personal recommendation would be Maxtor, but my experience is limited to Maxtor and Western Digital. Maxtor, for me, has been rock solid. WD has not been quite so, but no major problems.
Make sure you get 7200 RPM (less is too slow, more is better but definitely less bang/buck). 8MB cache is pretty standard. I've have seen 16MB cache around, but I don't know if it really matters to go that high.
Other than that, make sure you get SATA, not IDE. I'm sure people have different recommendations on specific brands of hard drive.
250GB might be pretty expensive. Unless you know you can fill this much up, I'd think about getting 160GB. That seems to be the sweet spot right now for good price, decent size, and a good expenditure of a hard drive mount. You can always upgrade later if you need, by replacing one of your current drives, investing in another mount system, or going for external drives (which are becoming QUITE affordable, by the way). If you know you'll need 250GB, then by all means go for it.
Good luck! I've seen great deals on various drives at Fry's Outpost.com... check
DealMac for some pretty spiffy deals. In the end, I'd just get whichever drive meets your requirements and is cheap, and not worry too much about brand.
Only thing I'd say is that if you are going to be using lots of drive space and you need it to be reliable and fast (ie: your paycheck depends on your hard drive handling large loads) I'd look into getting a RAID system. For the average consumer this is way overkill, though.