ok time for me to add my moneys worth here.. I also build and maintain PC's but not for a living. My PC that Lizz is using a lot I built from scratch and works like a charm still after 2 years.
The core components look good from here. For what you want to use a faster processer and a motherboard (the brains of the PC) is the best. It means that there will be less time loading and updating images. As for the operating system...Windows Vista is fairly new and not yet proven totally in the field. Myself personally i am waiting for about 6-12 months before upgrading to it. I want the bugs to be worked out totally and them upgrading. Perhaps you could talk to them and get yourself XP pro???
Memory - In my eyes the more the better so this is good.
Monitor - I always ask.. Do you have the room for it and is a big screen really needed?
Video Card - Nvidia is one of the best makers about (again a personal opinion) I always use them if i can. This is probably a very good match for what you want
HD - Serial ATA is very good, reduces read/write times and 350GB is great. Plenty of room for porn
Floppy Drive.. The only reason for this now is if you want to share small files with others.... It was important back in the days of ME for emergancy boot but now you can sort the PC out to boot from CD if really needed
Network.. Great if you have 2 PC's and one internet point. Means that they can both access it providing they both have a network card, 10/100 is the standard.
CD-ROM - Good call on the Duel drives, less wear on the drive for reading/writing and as has already been said means you can copy disks (if you need software for this give me a PM i have a HUGE stash of suitable about)
Sound Card/Speakers... Good option BUT i would have looked for a seperate sound card, that way if it fails you can replace just that and not have to look for a new motherboard
Office/Firewall.. Good call. But if there was an option for Norton/Symantec i would have gone for that. Everyone says that McAfee is great BUT norton tops the easy of use, reliable everytime. It may be a little more expensive BUT you can get bargains on it.
Service option - If anything is going to go wrong its going to be in the first few months. 1 year is more than long enough and by then most of the componants will be out of date anyways (usually happens in 6 months)
Might i make a couple of suggestions as well.
When i build a new PC i always run a benchmark test to check and make sure that all the components are working together. I also do a "burn in" session as well. I leave the PC on for 24 hours, not connected to the 'net and then a further 6 hours connected to the net. I use the PC as and when i need to during this. Again if anything will fail its during this as well.
Overall though I think you have a good deal there and if you have any more questions drop me a PM
Enjoy it