You need a mobo with 2xPCI-E 16x, that is, a motherboard with two full 16 lanes PCI-E slots. You don't need to get very worried if those slots are 1.1 or 2.0: 2.0 video cards are backwards compatible with 1.1 slots, and the gain you get from 2.0 is minimal. Of course, because 2.0 slots are current today, if you can buy a mobo with them, great.
You don't need to get a Intel X48. It's a great mobo, but you'll get stuck with DDR3, which does not make sense today in terms of price/performance.
If you enjoy the X48 chipset, you can find several models with DDR2 support.
Remember: you're not stuck to DDR3 because you went with X48.
Anyway, you can also go for a X38 mobo. It's also great and it's cheaper. Check Intel X38 mobo. It's compatible with DDR2 and it's a great mobo.