I'm planning on getting an Asus P5Q Delux Mobo w/ an E8400 and Nvidia Geforce GTX280. How would I figure out the best/fastest RAM that I should get with this combination?
OK "supported" is the key word. Any enthusiast mother board out there will run the fastest memory you can buy by using a memory multiplier (aka memory ratio or memory divider).
Keep in mind you did not mention if you intend to OC or what CPU you might be thinking about etc. You should be looking at CPU motherboard and memory as a system. not individual parts you can just slap in.
The "supported" bit is the manufacturer covering their weasel ass by hyping and selling boards (priced accordingly) they damn well know we intend to overclock but refusing to "support" us if we do. Basically, if you have an issue and call/email tech support and tell them you are OCing they 99% of the time will tell you your on your own and will not/cannot help. All they will "support" / "guarantee" is the stock Intel settings the chipset has been verified with.