I'd rather have an actual
nForce 4 Ultra board for the same price than an MSI nForce 4 board. If you do want one of the best overclocking boards out there, Asus isn't the way to go anyway. DFI makes two nForce 4 Ultra boards under $120, one of which can be modded to be an SLI board (any nF4 Ultra board with two large PCI-E slots can do this, in theory).
Don't get the copper XP-90, and you might have to get it somewhere other than Newegg. The aluminum one is almost as good and will cost half as much. You can also just go with stock as we've both suggested. It will overclock pretty well on stock (though I wouldn't count on 2.4GHz with a 3800, it's quite possible)
The 3800 won't
smoke that Dell system, but it will be comparable. If you want to OC now, go for it. Otherwise, you'll want to spend the extra on a 4400 or 4200.
I wouldn't bother with 4GBs of RAM, as 2GB is overkill for the vast majority of all applications. However, I would agree that getting dual 1GB sticks is a good choice. That particular RAM will perform very well.
I don't know what made you think you needed to spend nearly $400 on a 6800GT, but
here's one for under $290. It outperforms the X800XL in all of the benchmarks that matter (with the major exception being Battlefield 2, where it's more of a tie), and is more future proof as it has Pixel Shader 3.0.
Another big way to save money would be to go with a much cheaper case. The PSU in that case won't do (I already said this, yet you don't list a PSU...), and the case itself is rediculously expensive. I personally never intend to spend more than $50 on a case
after shipping, and I prefer to keep it close to $40.
Unless you're an audiophile, drop the sound card entirely. Pretty much all nForce 4 motherboards come with great onboard sound, and you probably won't think the difference is worth over $100 if you even notice it.
BTW, I'd pay
not to have Windows XP Media Center Edition. There's absolutely no purpose to this operating system, and when it comes down to it XP Professional is downright better.
And to answer your question, yes, the AMD system will last longer than the Pentium. Socket 939 has at least a year left before it even has another AMD competitor, while Intel seems to change its sockets quite rapidly. Besides, the fact that it is a Dell basically kills a lot of upgradability as the case and PSU are essentially useless with anything but each other.