Indeed. PCI-E 2.0 cards will work in PCI-E 1.1 slots. There are currently no cards that can break the 2.5GT barrier of PCI-E 1.1 . The reason why we have PCI-E 2.0 to market now, is to prepare for the break, as well as increasing the power draw to eliminate the need of a molex connector for certain cards.
There won't be any PCi-E 2.0 p35 boards I believe, you'll have to get X38 for that (if you go intel). Keep in mind though, If you plan to upgrade to Nehalem around their mid range release, don't waste money on a x38, you may as well nab a p35. IF you plan to nab a new high end video card around 1Q/2Q next year (rumor is, 9800 from nvidia will be around February), then you might want to wait and see if they have broken the PCI-E 1.1 bandwidth barrier. If they have, you have a reason to go X38. But, If you don't plan to nab a new high end, and want to upgrade to Nehalem, don't even worry, just nab a PCI-E 1.1....Nehalem will require a new chipset/mobo due to the new IMC, and QuickPath interface. So upgrade life for all current intel motherboards will be dead as of the end of 2008...Not much of a life span if you ask me.