Had to throw in my two cents here:Grad school is not a great time to be getting a puppy IMO. Unless you're one of those lucky, lucky people who don't have to work while you're going through school. And even THEN you're still going to be quite busy. I like the idea of an older, rescue dog who is already house broken. But whether you get a puppy or an adult, and no matter what breed or what size, a dog is an incredible commitment and takes a LOT of work: daily training AND exercise (even if you have a smaller breed and even if you did have a huge back yard), not to mention that they are more expensive than just the initial cost of purchase. In other words, i think if you're finding yourself having second thoughts, it probably isn't the right time.
I got my pup in grad school. Not really a pup, though - 7 months and already mostly housetrained. I think it depends on what kind of grad program it is; if school is your work, then I think it's ok. If you're working additional hours elsewhere outside of your program requirements, it could get tough IMO.
I agree completely regarding the rescue dog. Save yourself the hassle of a little puppy, particularly if this is your first. Housetraining is a big part of puppyhood, but there are plenty of other challenges to think about too.
The backyard isn't a huge deal IMO. I have a nice sized fully fenced backyard that the dog rarely uses, because it's still too small for out-and-out running. That's what parks, bikes, and sneakers are for.
Last point: If you're scared now, wait till you get the dog! In my experience, the first week or so is an utter nightmare. You are constantly asking yourself whether you can pull this off - again, probably a new-dog-owner syndrome. I pretty much couldn't eat for a week because of the pit in the bottom of my stomach. If you're scared, keep reading a lot (this forum is great! - the stickys alone will go a long ways) and you'll find out if this is really a good time for you to get a dog or not. Plus, if you decide to go for it, you'll already know all the tricks!