I dont understand

Ok im building my own computer. I have all the hardware but i dont have an OS. Now im a windows xp guy but i want to go to windows 7 because im a hardcore gamer and i want full use of direct x 10-11 and xp pro can only support 4gb ram and ima going to be running 8gigs of ram. anyways im like totally blind on what version of windows 7 to get. windows 7 home premium upgrade says i need windows xp installed first....i dont wanna have to install xp then 7 bacuse thats just ridiculous. I really dont wanna spend 200 flippin buck on the windows 7 home premium full...somthing that im missing here? and sorry iv never really messed with windows 7 at all so im not educated in this field yet.. also any version of win 7 will support 8gb ram right or is it dumb like xp and you have to have pro version?