New build/old build an upgrading ram from 4 gigs -> 8 gigs???
Have you gone into bios to see if it recognizes both models. (Had to reseat one of mine bebore bios would see it.
If you have not Verify both sticks are good then test one module at a time in both sockets (Use bootable CD with memtest86).
Is this an upgrade, gong from 2 x 2 Gig modules (4 gigs) -> 2 x 4 gigs ( 8 gigs).
I have the Gigabyte P55-UD4P. went from 2 x 2 (4 gigs) -> 4 x 2 (8 gigs) -> 2 x 4 + 2 x 2 (12 gigs). Only problem was in making good connection at socket (My ram Ripjaws)
added
Link to iso of memtest86:
http://www.memtest86.com/download.html
Use iso function to create iso image. With win 7 built in burner I think you can just drag the iso file to the CD/DVD drive.