There arn't any socket 478 boards that suport PCI express, but why do you need socket 478? Are you upgrading something? Socket 939 aren't any more expensive. Plus its AMD, I'm assuming your a gamer because you want PCI-E, and AMD's tend to do better in games.
Well , since i ave 100$CAD right now... me a good price for a cpu is round ~400$CAD.
~150$CAD for an motherboard...
Im upgrading thought , last year i build that pc , now i look for go better with an 7800...
I just ask for see the solutions to that upgrading:
1.upgrade to S939 mobo+cpu+video (Asus A8N/3800+DC/7800)
2.upgrade to S775 mobo+cpu+video (Asus ???/3.4ghz/7800)
3.stay on S478 mobo+video (Asus ???/7800)
Out of 1 and 2, go for 1, AMD's are have better bang for your buck in gaming. I'm not sure option 3 is posible beacause I dont know of any socket 478 PCI express motherboards, but I'll have to look into it more.