memtest! That's a free Linux tool for stress testing memory with cd images available now over the original dos type floppy version. Memtest86+ - Advanced Memory Diagnostic Tool
sees the latest version there. The alternate site even seeing a premade cd for cost and shipping is available at Memtest86.com - Memory Diagnostic
When starting memtest also seen now as a boot option in Vista you let the tool run for several hours to see a series of stress tests repeat over and over which will report any faults found with the memory installed. Another quick method to see if there any immediate problems noticed would be removing one of the two dimms and running memtest on one at a time in order to detect if one is faulty.
The specifications for the Asus board on that model show that speed and DDR2 667 memory are the standard and should run without any problems unless you simply ended up with one dimm being defective. PNY memory is far from being one of the top brands while defect free memory by them will still work. Memtest and trying each dimm by itself would be the thing to do now however.