Sorry, I didn't have much time to post in a thorough manner earlier.
Puget's website is: http://www.pugetsystems.com
I see they have a preconfigured 64-bit dual Xeon workstation listed on their home page now for about $4,200.
Since you do such heavy multitasking, Photoshop work, DVD-burning, etc., I was going to suggest that you get a dual processor-based system, so I think you're wise to consider that.
I've researched custom PC companies quite heavily over the last year. Although I ended up building my own system, I spoke extensively with the folks at Puget. I'm very particular about components, and they were happy to accommodate me. They can build anything, from your basic budget PC to a high-end super system, like what you're proposing -- and they can do it quickly to boot. You'll see Puget is also conscious of the growing demand for PCs that are powerful yet quiet, if that's at all important to you.
I also talked a lot with Tantus and even got as far as a quote, which I thought was reasonable, given the level of customization, warranty, personal attention, etc. Again, don't be put off by what might appear to be a limited selection of components at Tantus. They were more than happy to get me anything I wanted. Since you know what you want your system to do, I'm sure they'll be happy to work out a configuration tailored to your needs. Also, it may not be apparent from their website, but Tantus can do case mods for you if you need them or want them.
I hope that's a little bit more helpful!