Allright, so something like
Lightning Review: Belkin 2-in-1 HDMI Switch ?
So i can insert both DVI-->HDMI cable + the Xbox 360 HDMI cable into that device and buy another cable that is HDMI-HDMI and im set

?
Edit : I've taken a look on some sites, i see many different DVI-HDMI cables. Like here
allekabels - 17500 elektronica artikelen online - allekabels Couldn't figure out which one to choose
( i know it's Dutch but most of it is English on there

)
Edit 2: Would be smart to buy a new videocard too, as X800 Pro isn't suited for HD etc. I have found this relatively cheap gfx card, which still does well and actually has an HDMI/DVI out, so that would be perfomance improvement+not worrying about DVI-HDMI cables.
Club 3D GeForce 7600GS - Review - De beste VGA kaart tot nu toe