Option one is the safest bet for you, but the way MS plays the game it might not be available from the vendor. I wouldn't go for W7 before the first service pack is out - well, any windows product for that matter. So if you *HAVE* to use Vista, you will need to be wary.
Look up online forums and see if the particular hardware config (MB, processor, display card, sound) have had problems with Vista. If you can't see too many complaints, go ahead with it - on the right hardware, Vista runs beautifully. I have an Acer Laptop, with 3 GB RAM and a 1.86 C2D/centrino, and I have had absolutely no issues with Vista SP1. Seems Acer sorted out their drivers very well before release. Not a single hang-up, unwarranted restart, BSOD since September. I haven't shut down the laptop since December (when I wasn't around for five days) as I only use Sleep mode. I have at times run Firefox, MATLAB, Word, Excel, PowerPoint, WMP and a PDF reader all open at the same time (What can I say? The workload at university can overwhelm me more than my computer!) with an antivirus scan running without lag. XP *may* have been able to handle all at the same time, but I doubt and I had no option anyway.
But, one anecdote is not the same as concrete data - if Dell haven't yet sorted the drivers especially with new hardware (the C2D was not new when I bought it) you will not enjoy it.