A 90-Watt adapter will work just fine with any E1505.
I don't know if you actually get any benefits from doing this over and above what the standard adapter provides.
Going through two floors might be hurting your data rate. However, it might not really affect games, as their bandwidth requirements are not as high as you might think. I'd give it a try and see if they're playable before worrying too...
The Inspiron E1705 is an awesome gaming rig. If sturdiness is your primary concern, you could look at IBM, but their machines aren't typically aimed at gamers.
Whatever you decide to buy, just make sure that your parents insure it...
Your temps are probably fine, and your card probably does underclock when you unplug the main power. You can adjust this in the nVidia driver settings menu.
If your Linksys card is only 802.11b then that would explain the difference. Your new one is probably 802.11g which has much better range.
Oh, and I'm going to go ahead and assume that you've got your neighbor's permission to use his...
