Re: From what would I benefit more?
The P35s are just as good for OC'ing as the X38/X48 but I think the Vdroop is slightly worse. (I think I remember A true folder saying the Vdroop on his P5K was worse than on his new P5E) I don't think you'll need DDR3 until Nehalem and like you said, you'll need a new mobo then anyway.
And PCI-E 2.0 offers no performance advantage over PCI-E 1.1 and by the time it does, you'll have upgraded probably.
I'd recommend the Abit IP35 PRO, but if you don't want to spend that much on a P35, then the GB DR3R is my recommendation.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.