I went through the same decision process beginning last December and went with the ra820g in February, and am very happy with choice, however, it depends on what you are looking for. I wanted the "near silent operation" of the Sony and am not doing anything (e.g., high end gaming or graphics) that the vaio does not fly through. That is to say, specs may be a little better overall on the 8400 (e.g., DDR2 RAM at 533 MHZ is available), but not to the extent that I expect to ever notice. The default graphics card is even a little better on the VAIO I think (300 LE instead of 300 SE on the Dell), although with the 8400 you have the option to upgrade the graphics card if that is something you need.
What was also important to me was to have the Audigy 2 ZS soundcard installed (by a technician on the day of purchase so I could keep the warranty)--to me it seems silly to go with integrated audio and it's fair to say that the Sony speakers are mediocre. I think one point that may be overlooked is that you can modify the Sony to your specifications via the Geek Squad at Best Buy or whereever (I bought mine over the internet from jrworld) and even possibly have more choices; for example, with regards to ATI graphic cards, Dell lets you choose between the X300 and the X800; however, with the Sony you could swap out the X300 for an X600 or X700 and have it installed on the day of purchase without voiding the warranty.
The other obvious consideration would seem to be the tv tuner card, which I confess is something I am not planning to explore till later; but from what I've read, the Gigapocket card on the Sony (if that's the right way to say it) is not as good as Hauppauge cards (which I believe are available on the Dell), about which the word uniformly has been quite superlative from consumers.
Also picky things like USB inputs may matter to you, a number of folks have complained bitterly about the angle of entry on the front of the 8400.
Also, the VAIO has to go back to Sony for repairs I'm told due to propietary parts whereas the Dell uses standard parts that more likely can be fixed in-shop. So unless you plan to have a back-up computer as I do, the Sony could be less convenient to fix.
So as always but nowadays more and more as the specificity of technology grows, it depends on the feature set you want. The wisdom a geek once shared with me, however, that I still live by when choosing electronics is to decide your priorities and give preference to a smaller feature set that clearly excels in the areas most important to you over a swiss-army-knife type of device that does everything but nothing more than moderately well, and may even miss the bus on what matters most to you.