Hi Dark! As for the prices on items, there is currently a shortage on RAM, and those prices have been trending up, however graphics cards have been dropping (finally!)!
As for the Mumbo PC, I'm not familiar with AMD processors, so I cant say for certain, but the price does seem reasonable.
A monitor will depend on your graphics card. You can see the specs on the manufactures sites. When looking for a monitor, FPS (frames per second) is expressed as Hz (Hertz(sp?)). So a 60hz Monitor can only show 60 FPS.
edit As part of making your purchase I still recommend price checking with other custom builders, as well as a site called New Egg (i don't know if I'm allowed to post links) which is like the Amazon of computer parts