I totally didn't believe I miss that, The only thing I can tell between the Platinum and yours is that the Platinum has 1 external SATA and comes with a little more SATA wires. No I don't believe it's worth the extra 36. It's basically the same thing in spec other than the nice heat pipe. I would stick with yours for the budget.
Wide screen will work perfectly in XP, actually it looks very nice with wide screen. That Acer I chosen for you is 1680 x 1050, much better than your Han 1440 x 900. Plus the Acer is 800:1 and it's only $10 more. I recommend you get at least 1680 x 1050 to enjoy the wide screen. Your 8800GTS will perform very well also at that resolution.
Your case is OK, with the back 120mm fan as an exhaust. However there should be one more fan to blow cool air into the case. I don't really like the style, I usually buy case that looks nice with room for expansion but that's up to you to choose the case since we all have different taste so I can't really help you there
I also wanted to add, if you are patience that Q6600 will be $266 in just 20 days. If you swap the E6600 with the Q6600 on the 21th you will kill that Gateway. The reason I told you not to buy the Gateway is:
1)For a Q6600, that Gateway will have to sacrifice a lot of it's hardware to be under $1300.
2)I would not sacrifice hardware just for a Q6600.
3)That 8800GTS will blow that 8500 away on the Gateway, plus after the 21th, your system will be close to the Gateway's price but you have better hardware and an 8800GTS
5)If you need more juice you can opt for more watts, that Rosewill will get the job done but when you start adding more parts it might not do so well because of your 8800GTS.