I don't think anyone is calling you a liar, but there does seem to be some disbelief.
So far we have....
Memory 2 2GB sticks for $98, 8GB = $198
(what brand should I be looking for?)
22" monitors @$150 each for $300
(brand of model number?)
That's half the $799 you mentioned you built an equivalent MacPro spec'd PC for. Even if you just priced the remaining parts at $50 a piece, unheard of pricing, you would exceed the $800 by a large margin.
2 video cards w/1GB each
Dual Ethernet card/1000 Base-T
Hard disk 320GB/sATA/7200
Under those circumstances, you shouldn't be surprised to find skeptics. But hey, list out your components, specs, cost, manufacturers and throw a little pooh pooh in their direction