Did a quick search on Newegg and couldn't find anything better than an X700 low profile, but if you really need more power, you should definately look into a riser card.
You plug it into the PCI-E slot, then plug your video card (whichever one you decide to get) into the riser card so your video card is parallel to your motherboard. If you go that route though, overheating might be an issue depeding on the airflow in your case. It's definately something to research.
EDIT: So it turns out that I'm an idiot... that riser card I linked you to is only compatible with the case that is displayed on that webpage. Anyway, I'm sure if you look hard enough, you can find a riser card to fit your needs if you decide to go that route.
EDIT again: Here ya go... this one doesn't say anything about compatibility issues.