New GPU that will fit GAEP35DS3L (thickness)

So I'm rebuilding my young son's PC because of his sudden interest in gaming. I just installed the GA-EP35-DS3L motherboard with an e8400 Wolfdale. I installed a new PSU (700w OCZ gamexstream), and now I'm wanting to upgrade the GPU. As you can see it still has an old Sapphire X1950pro 512mb pci-e in it.

What I'm worried about is that this card is already right up against the SB heatsink (though part of that is a reflection on the plastic of the card). I know some of the newer cards are even thicker, so I wanted to get advice here on a good card that would fit? Length is not an issue as this card is already huge and there's room left behind it, I'm more worried about the thickness. I may be worrying too much but I thought I'd ask before I order.

Any recommendations for a card would be greatly appreciated. Thanks a lot.