i'm in need of buying a new desktop PC, and i would like some advices from you techy people

_budget: let's say 500-600€. I can think about something more though if it's really convenient, but i would like to stay low.
_usage:
- the most resources-hungry thing i plan to do is playing GW (perhaps GW2, almost certainly no other big games).
- I don't do video or image editing, i don't use 3DSM etc.
- I won't overclock, nor likely update my hardware in the next few years.
- As far as the OS is concerned, i'm wondering if i'm better off buying a new Win7 64bit, or going on using XP 32bit. What do you think? Is the cost worth it?
- I won't use multiple video cards. I'll probably use only one HDD, since now they come with 500GB/1TB, and i've already got an external usb disk with 500GB, which is more than enough for me.
_screen resolution: 1680x1050 since this is the max GW resolution iirc (i have to buy a new monitor, maybe a 18" or 20", i don't think i will need a bigger one).
What's your suggested setup? (Also sorry if i didn't provided enough information, feel free to ask me in case).
I've got no experience in building computers. Should i buy the individual components and try building it on my own, despite my noobiness, or would i be better buying a prebuilt/custom built in your opinion? I live in Italy (north-east).
Thank you all for your help
