I had the same problem, and while I ended up decideding on the i9300, that was partly due to a dell aus sales rep assuring me that the i9400 "would not be released in australia anytime soon" which I didn't put too much faith in as at the time it had been out in europe and the us for a while. Sure enough less than 2 weeks after the conversation and a few days after my laptop was delivered the i9400 was up on dell.com.au.
Anyway had been following dual core for a while and really wanted the multitasking performance and also to futureproof components and upgrades as much as possible.
However, considering that I could still run photoshop with no real issues on my i8600 - 3ds max also ran fine as far as I am concerned, it lessened the need to get dual core for these reasons even though you would get much better performance with a dual core processor. When it comes to multitasking and system responsiveness though, single core not HT cpus lose out big time.
Keep in mind the thing that will determine when you have to next upgrade your laptop (barring theft, or it blowing up) will probably be the video card. The 7800 gtx is a fairly recent and cutting edge card at the moment and outperforms the 7800 vanilla significantly going by benchmarking that has been done, and while there is a possibility of dell releasing an upgrade later, it is not guaranteed. At least with the XPS, you are guaranteed at least 7800 gtx performance and as games start becoming more gpu limited in these systems in the future, this is where the bottleneck will be, and not whether you have a single or dual core cpu.
Of course if you anticipate upgrading your laptop within a year or year and a half the vid card probably doesn't matter as much, both will get the job done till then.