I hope anyone can help me?!?!?!?!?!

hi folks,i have got an issue that i cant seem to fix,i joined this site cause it seems to be a reasonably common problem in these forums. and have done almost everything here and still cant resolve it. so a ittle background first. for the past few months i have been having boot issues,my computer would turn on,but the monitor would not power on,i would have to reset several times before the monitor woud turn on. on another not if i was making any changes to my computer,and reset system from start menu,it would come right back on,but if i did a shutdown,it would do the same thing and it would take forever before it would come back on again. now for the present.. i am a fairly avid gamer and when i purchased my computer last year(dell XPS 625) i had it custom made to last a good while..it has 4g ram 750w power supply,the video card was a ati radeon 4800,and i played everything just fine,even when it started messing up i still had ultra graphics set and it was smooth as silk. now i am at a loss because my monitor no onger comes on no mmatter how much i try and reboot,screen just satys black. computer powers on,led's come on,all internal fans appear to be working. i was told by dell it sounded like my video card burned up so i purchased a new HIS radeon HD 6770 iceQ X. came in the mail,and was so anxious to play my games again,and reinstalled and hit the power button.screen still black. i have reseated the ram,i have chanced pci slots,i have reset the cmos,reseated all connections and still nothing. everything comes on but monitor. i have also tested on three differnt monitors and i know they all work. please please help with this,if there is any info i left out let me know,and i can give more. i just dont want to assume its the motherboard until ive tried everything. this site has helped alot of people with thier issues,i hope it can help me with mine..thanks