With x2a ... I know it sounds like a dumb question: but can they interfere with the program? I.e., do they use "a lot" of resources?

I don't use a wireless keyboard (although I'd prefer to). I just built a gaming/X2 system and there were only a very few gaming keyboards that were wireless (I ended getting the (wired) Sidewinder by Microsoft).
 
I found this thread about keyboards which might have some useful info there......(it seems like wireless keyboards should work just as well as wired (I think it's just the distance/reception that might make a difference). It's always good to have a PS/2 keyboard as a backup in case anything goes wrong with the wireless.
 
http://forum.cakewalk.com/Sonar-X2-keyboard-m2744250.aspx

I use a "newer" wireless Logitech mouse and keyboard combination with a usb nano receiver.
The battery life is incredible!
Before that, I used to use an older Logitech mouse and keyboard combination with a "puck" as the receiver.
One thing I did notice is that when I used the older set, my dpc latency was low.
Once I started using the new usb nano receiver combo, my dpc latency is even lower.
My dpc latency is down to a consistent 35 with an occasional spike at 50.

Hey guys, thank you for the very helpful comments! I'm thinking of going wireless. That's why I asked. Sounds like there's no reason not to!

I use a wireless logitech keyboard and mouse. Rock solid with no serious lag issues. When I get them I know its time to change the battery.

I use Microsoft Wireless Desktop 2000.  I also purchased one for work.  The layout is a little different, but once I got used to it, it works fine.  If you buy from Amazon they have two versions, Business and Retail.  The only difference I found was the price. 
 
-JJ

Are these wireless set up you guys are having success with USB?

Yes.  The USB dongle is in the back of the computer under the desk.
 
-JJ

Wireless Microsoft Arc keyboard with USB dongle. No problems other than the keyboard isn't as responsive/fast as the previous wired MS Natural one.
 
No dpc latency issues/problems at all. Wireless keyboards/mice don't use the networking stack (which is where the dpc issue lies); as far as Windows is concerned they "look" and work the same as USB ones.