Some thoughts/opinions
On the system choice...
1. Don't do Dell. If you can't build yourself with standard components, have someone else build it for you with standard components. You may not initially regret the decision to purchase the Dell, but eventually you will.
2. Don't need a physics card. From what I have read, nVidia has submitted their version 177 drivers that will bring PhysX technology to series 8xxx and higher GPUs for WHQL certification. If that is true, then they should be released in the very near future and you will have your PhysX for free. TBH it is not that important. With the exception of UT3 and a couple of other minor titles it has not really been implemented that much. Intel has bought Havoc, and is pushing their version of physics coding. Needless to say ATI (naturally) is leaning to supporting that standard. At this point in time, physics implementation is still spotty and with Havoc/PhysX there is no clear standard that programmers are using when they do choose to incorporate it.
On the component choices...
3. The power draw on that system spec is going to be pushing near the upper range of that PSU. In addition, that unit does not have active PFC (power factor correction). I would not use that unit for those components. I would recommend a unit from PC Power & Cooling, OCZ, Corsair, or FSP that features active PFC. Bare minimum of 600w and 40 amps combined 12v circuit. Any on this list will meet those requirements...
Newegg.com - Retail, 801 - 900W, 701 - 800W, 601 - 700W, Active, PC Power & Cooling, OCZ Technology, Inc., Corsair Memory, Inc., FSP Group (Fortron... My personal favorite brand is PC Power & Cooling although any on that list are acceptable.
4. OS - Vista Home Premium 64-bit (what minor additional features the Ultimate version offers are not worth the price premium). 64-bit version in order to gain the full use of your 4Gb of memory.
5. That motherboard does support your listed processor provided the BIOS is flashed to version F3b or later. Ensure that your builder covers that item or you will suffer performance issues. I have never owned or built with Gigabyte mobo so I can not speak to their quality/design, but I know Pedro thinks well of them. If you are set on Gigabyte you may want to consider the GA-EP35-DS3R (rev. 2.1) if it is available and not much more expensive. It appears to have a bit more flexibility and aligned more for the performance market.
6. With regards to the memory the only item to be concerned with is the large heatpipe/sinks on the dimms. Those can create clearance issues/obstructions with some mobo layouts and/or CPU heatsinks. So long as your builder assumes responsibility if they don't fit, then no problem. From a brand perspective, OCZ is bullet-proof for performance/quality. Always my first choice in memory.
7. If you are going to the expense to purchase an X-Fi sound card I would recommend one of the versions with the 64mb of on-board RAM. The price difference between the version without (XtremeGamer) and the version(s) with (XtremeGamer Fatal1ty) is only about $45. There is a PCI and a PCIe version. I'm not certain about clearance issues with the single PCIe X1 slot on the mobo you list if you choose the PCIe model. The mobo model I listed has three PCIe X1 slots so it should not be an issue on it. If you choose the PCI model then the clearance issue is not a concern.
8. Hard drive size is your option. I am old school in that I don't believe in dumping every last piece of data on my hard drive. If it isn't something I use regularly I burn it to CD/DVD and save space on my drive in order to maximize seek/read/write performance. I don't believe a huge HDD is a necessity. It is a convenience, but when (not if, but when as all hard drives will eventually fail) my hard drive does die/crash/fail I don't lose ALL my data because it was in one place. The other advantage to having a hard drive only as large as reasonably necessary is that it minimizes the cost of the external drive that I use to back up the data that is on the hard drive. To me a 500Gb HDD is more than large enough for the average user if they practice reasonable data management. I have less than half that capacity in my system and I have never run out of space. Vista is considerably more of a storage resource hog than XP but not unreasonable. I run Vista Home Premium 64-bit and my OS partition is only 50Gb in size and the OS takes about 25Gb of that with installation and SP1. That leaves me with 25Gb of space for drivers, registry files, etc..... Most of them are small and not going to need much space. My programs, pictures, music, etc... I store on separate partitions of the drive(s).
9. You won't be able to appreciate all the abilities/features of the X-Fi series of sound cards with just a 2.1 speaker set-up but you will still reap the system performance benefits of the on board DSP and RAM (if you choose a model with on board memory).
To summarize my long-winded ramblings in regard to your questions:
Go with the custom build, Vista Home Premium 64-bit OS, no PhysX card required, but consider a different power supply (one that features active PFC). Sounds like you will have a very nice system when completed.
