Personally if your willing to pay extra the only way to go is build your own.
Sprogget seems to be the man if your after a multimedia experence.
I had a look a month or 2 back at building my own but discovered that the locals in my part of the world just can't get the decent stuff as there is no market for it.
If you haven't built one before it really isn't as scary as you think.
Its all to do with planning. The putting it together will take you a day, then loading the OS another day.
Personally i would go for....
Decent case with not less than 750 PSU
quad proc AME
mobo without a onboard graphics or sound but does have hardware RAID.
Medium level Soundblaster card of some form
Medium level Nvidia graphics card with as much memory as you can afford.
Memory: Fill it up but if you want it working and then upgrade later just us the max size the socket can take so you don't fill the banks up.
Disks: CD/DVD burner etc Couple of HD one for OS one for data you can go to 3 if you want to use the raid as a backup solution.
OS: Win7 64bit. home
Network: The mobo will prob come with 10/100 but you might need a wireless card
Once you get into it its acutally quite good fun and you will have alot better machine than you can buy off the shelf but it will cost a bit more.