I would suggest some mods to your parts:
1. Check the specs for the max memory supported by the motherboard, and the allowable configurations. Ideally, you want a motherboard that can support at least 8 GB of memory, and if it not much of a cost bump, try for 16 or even 32 GB support. Also, each memory strip should be able to be at least 4 GB in size, (8 GB is the other size if going to a board supporting 16 or 32).
2. Try to get at LEAST 8 GB, which only runs around $40 for 2 4 GB strips. If the motherboard supports more, you can pick up either 4 4 GB strips, or you could get a single 8 GB strip and add more later it needed - or get 16 GB. 2 8 GB strips run about $52 or so these days. Memory is one of your best friends when setting up a machine for audio processing, such as recording with Sonar.
3. You can buy a 2 TB 7,200 rpm SATA 6.0 gps hard drive these days for around $99, so I wouldn't even think of buying a single 250 GB drive. You should try to get 2 disk drives for your new machine in any case, 1 to contain programs and the OS, and the other for samples and storing audio. Splitting partitions on a single drive does nothing for performance (is however helpful for organization and backups and such). Splitting it off to a 2nd drive will help things load faster and therefore perform better.
4. I am not sure about that processor, so I have no advice one way or another. I thought that the Xeon processors were for servers, and if that is the case, I would recommend a re-think on your part. I believe that an i5 or i7 from Intel, or an AMD Phenom II 965 quad-core black edition would be a much better choice. They would be better suited to audio processing.
5. The power supply sounds a bit small for what will be asked of it. I see lots of systems with a minimum of a 500 watt power supply. I currently have a 1000-watt one, but I think perhaps a 650-watt or better would be a better choice than the 425-watt one in your specs.
6. I just re-read your comments about alteration on memory, and the adding of an additional drive. Good choices.
7. It has been my experience that vendor companies, like Dell, sometimes skimp on the quality of some components, such as the manufacturer of internal components, so check out the maker of all of the internal parts. I just build my own, have done so for the past zillion years. Look at the cost of all of the individual components from a place like
www.microcenter.com, and compare that to the cost Dell is charging you, and you may end up re-thinking how you approach getting your new machine.
I hope the above helps - there are lots of opinions and choices, but basically you want a quad-core processor (Intel is a bit faster than AMD, but AMD gives you good bang for the buck), at least 8 GB of memory, 2 hard drives, and some middle of the road or better video card, with a power supply of I would suggest 600w or better, and you should be fine.
If you have any money left over, you can also look at picking up a second monitor - you can buy a 32" HDTV that would work wonderfully for a computer monitor, for around $200. You can look at those on the site I provided above, and also like Best Buy or Walmart or wherever.
Bob Bone