Switch the Diamond 4870 to the Visiontek one.
NCIX.com - Buy Visiontek Radeon HD 4870 512MB GDDR5 PCI-E 2XDVI-I Video Card - 900244 In Canada.
If you're gonna crossfire it, get the 4870X2 instead. Also, the P5Q Pro is PCI-E 2.0 so even though it runs at 8X in crossfire mode, it should offer the same bandwidth as PCI-E1.0 16X. Bottom line, I say stick with the P5Q Pro.
Since you're using a 22" LCD. I don't see any reason for you to crossfire.
For your sound card, I'm not sure about the HT Omega. You're on your own for that. And I say try integrated sound first before going for a discrete sound card. What speakers are you going to use?
Otherwise, it looks like it'll be a great build.
Where are you getting the OCZ Vendetta 2 for $30?