The difference between X38 and P35 in terms of performance is completely negligible. In almost all benchmarks
and OC testing, both boards come out on top, with little to no difference between them.
Both boards support quad-core, and both boards have support for Penryn. If you're getting a P35, make sure it has full support for the 1333MHz FSB - that's the bare minimum for Penryn. All X38s support 1333MHz FSB by default, so no worries there.
Only benefits an X38 has over a P35:
- 2x PCI-E x16, full speed. So you get better SLI or CrossFire, but not by too much.
- PCI-E 2.0 support. This is a fluff, there are still AGP cards being created, even in DX10 flavors - it's unlikely you're going to have to worry about PCI-E 2.0 for another 5 years or so
That's pretty much it.
Though I would recommend the Gigabyte P35-DQ6 over the DS3; you get the best performance/price ratio - especially since it's like a 30 USD difference between the two, or something around that. But both are good boards
- and more importantly, both are Penryn compatible, so you'll have plenty of room to expand in the future. Only reason to go with a DQ6 is if you're going to CrossFire.