I also have SFIV arcade edition on Steam, but haven't gotten the foundings yet to buy a controller.
There are ways to play with a keyboard that actually aren't horrible. I actually prefer a HitBox-setup on a keyboard(that is, using Space as the Up-key) above a regular pad.
That, or a controller converter for a Xbox/PS3-pad doesn't cost much afaik.
I don't play that much SF4 myself though(prefer TTT2 and SFxT), and I'm fairly bad at it. I also get framedrops and stuff on PC. :V