First, my bad for missing the Lima thing in the specs post.
I have to agree with the Phenom II X2 550 argument, if it's supported. That or an X2 Kuma 7750/7850 would be a better choice than the X2 6000+. 2 MBs of L3 cache works wonders over none at all, even at their lower stock-clock speed. You should probably check with Gateway (maybe even ECS) for a BIOS update to support these "newer" processors.
As for power concerns, there are plenty of excellent 400-450W PSUs available at Newegg.com for 30-50 dollars. (I just bought an Antec earthwatts EA430 for $49.99.) Many (not all) of them are of newer design, which means they're more efficient and supply a solid amount of 12V power, which is exactly what more modern PCs need.