They're a good place. They have an actual store in Tulsa with a ton of equipment - I have been there and purchased in person as well as online. The sites are different because MegaWatts caters to local customers, while MegaMacs is for online orders only.
I was in their Tulsa store last week. Imagine anything PPC related times 100 and you pretty much have an idea what the place is like. A few years ago they had literally hundreds of G4 and G5 towers sitting on the shelves like stacked dominoes. They've thinned most of that out now... but their stock of PPC stuff is still impressive.
They're nice and knowledgeable folks. Definitely worth a stop while in Tulsa since you don't come across vintage Mac retailers everyday. I do think their prices are fairly high in relation to what the stuff is worth on the market today, but if you're looking for something Mac that's hard to find... they've probably got it laying around there somewhere.
Their current pricing on the 2009 Mac Pros looks pretty good. I'm very tempted to buy one myself for a modest but significant upgrade to my 2008 Mac Pro (much cheaper RAM, ahoy!), but I have other things I need to save for / spend on at the moment.