Some motherboards with integrated graphic cards can in fact accept actual graphic cards too. Check if they have the correct slot, and you should be good.
About OS, you can always format the pc and install whatever you wish on it. In fact, I actually recommend it if you have the install cd for the OS you wish (or want a free OS). Call it paranoia, but in general, I prefer not to trust others with installing stuff in my pc if I can possibly avoid. If you don't do it, and it comes pre built, most likely all space will be already partitioned, which, unless I'm wrong, makes it much harder to install another OS on it. If it comes with more than one partition you might be able to format one of them and install it there.
On yet another note, it's really easy to build your pc from parts. Get a chassis, screw the motherboard on it (both have pre drilled holes for it, of course, just match the patterns/size). If you can swap parts the rest is the same, just plug in the parts. The only part that's a little annoying is the processor, specially if you get one with tendency to heating too much (specially if you live in a very hot area), where people in forums generally recommend you get a better fan, and in particular a better thermal paste. Still, as you seem to be building on budget, it's very unlikely to be an issue, so just get a processor that comes boxed with a fan (and generally also with the thermal paste already on the fan).
And a final note, about windows seven, I've not got it yet (although will be probably getting it later this year), but I've heard pretty nice things about it, specially compared to vista.