HIS ATI Radeon HD 4850 on a Dell e510

ok, so I've been planning on upgrading my computer for a while and im planning on getting an HIS ATI Radeon HD 4850 1GB graphics card (http://www.newegg.com/Product/ProductReview.aspx?Item=N...) for it. I already have a PSU upgrade in mind to a 600W but thats just a fun fact.

My main concern is that i have been reading all over the internet that the dell e510 doesn't take a dual slot PCIe x16, and that it has to be a single slot. Is this true? or it doesn't matter whether or not it's a dual slot just as long as it has space for a fan and extra room for the card itself?

If this is true and I can't buy the HD 4850 then, then whats the best graphics card I can buy that is a single slot and will fit in my motherboard?

P.S. I'm willing to no more than $140 for a graphics card that fits my criteria.