Is the Antec900 too small?

It's not too small, only if you upgrade to 480 you might have some issues. Also if you add another video card, might bee too much heat inside that smallish case.

This hdd is better:

http://www.amazon.com/Samsung-Spinpoint-Cache-Desktop-H...

the antec 900 is rather big case if you ever see it

It will fit, but it will be tight with that long of a GPU and multiple HDDs' worth of power cables and SATA cables coming from the drive cages. The Nine Hundred is a typical mid-tower ATX case and the HDD cages sit directly in front of the MB, so the cables interfere with very long GPUs (been there, done that.) I'd recommend a great big old full-tower case if you want to have plenty of room.

how tight would the inside be?

Its a bit snug but you might be able to do it. Just remember its only a mid-tower case and you can get a Coolermaster HAF 932 for the same price and it has a lot more features and is HUGE.

Tight enough to make it moderately difficult to work inside the case with the GPU in place but not too tight to prevent the GPU from fitting in the case. However, I admit I'm personally biased towards large cases with lots of room so it is easy to work inside the case.

In case you wonder what I consider "large," here's the case I got for my upcoming workstation build.*




The motherboard in the latter shot is an old micro-ATX Socket 370 board I put in there for reference, in case you are wondering. You won't need something quite that size to fit your GTX480, though  


*Unfortunately that build is a little bit away from happening due to lack of funds. A case like this was exactly what I was looking for and it took me several months of looking to find anything like it, so I snapped this guy up before the seller sold out. It should handle a nice SSI EEB/EATX or MEB/SWTX motherboard and a good-sized disk array just fine.

If you are not going to put more than 3 drives in the case, you will have room. Put the 3 drives in the lower bay, and remove the piece of black plastic from the top of the upper drive bay.

See the pictures in my case mod article.

Antec 900 case mods for cable management:
http://www.tomshardware.com/forum/264112-28-antec

Look where the PCI power cables loop. That's where that black plastic fan mount used to be.