If I were purchasing the 17" MBP from the Apple store, I would be inclined to only upgrade the screen to the HD matte screen. The text on the screen may seem small to you know, but once resolution independence comes into full force (which should be within a year) the extra resolution will be very desirable.
Upgrading to the 300 GB HDD may actually make your computer slightly slower when compared to the stock 250 GB drive as it is a slower drive. I don't think the extra 50 GB in density are enough to completely mitigate any speed differences.
I have mixed feelings on the processor upgrade. $250 seems to be a bit of a premium for just 100 mhz in speed. You would get more bang for your buck by upgrading ram, but not at the time of purchase. You can get the same speced ram from places such as OWC or Mushkin for significantly cheaper than what Apple will include on a BTO.
Hi welcome to the forums.
Naturally, it comes down to personal preference, but I prefer 15" models because they're more portable, plus you save a little money (or have more money to spend on extras).
That is also a very good point. If portability is a concern (and even if it is not), you can buy a very good (but not a cinema display) external 20" LCD for the difference in price between two otherwise identically specced MBPs (the 15" 2.5 Ghz and 17" 2.5 Ghz)