PCI-E(number) is compatible
PCI-E IS PCI Express

With what you have I would recomend one of these 5 Mobos

P5N32-SLI SE Deluxe

P5W DH Deluxe

P5WDG2-WS Professional

P5B Deluxe

P5B

I had a P5B on my last pc with a e6400 now I have a P5W DH Deluxe with an E6600 . Its great. Has Wifi and a great sound card. As far as USBs go it has 8 x USB 2.0
2 x Firewire 400

Core 2 Duo Platforms From Asus - A Top To Bottom Comparison

With what you have the extra $100 is well worth it. If you want to run dual vid cards though I will recomend you another mobo.