Installing XP on newer hardware may be a problem -- especially with a laptop. Newer video cards may not have drivers for XP. But at least with a desktop you can choose a video card that works with XP. And then there are all those other parts of a laptop that probably won't have drivers for XP (unless you buy a laptop that's over 5 years old).
Many new laptops that claim to have video cards that look good specswise use "Optimus" technology, which piggybacks Nvidia graphics on top of Intel graphics. There are no XP drivers for Optimus, so you end up with the laptop only detecting the nearly useless (for games other than casual games) Intel graphics. Even older games may have problems with Intel due to driver incompatibility.
So if you want a laptop with XP, I'd recommend buying a used one that is pre-Optimus -- and check the manufacturer website to make sure they have XP drivers for the model. The alternative might be to buy a high end gaming laptop that only has a Radeon or Nvidia video card and doesn't use "Optimus." All but one of the laptops I saw at at the local Best Buy use "Optimus" (which is a power saving technology), and that's all you're apt to see in a pre-built laptop. The one exception at Best Buy was a laptop with a low end Radeon that had no discrete video RAM, which wouldn't be much use for gaming.
If you've got the money, you could go for one of the non-Optimus models at a boutique shop like http://www.xoticpc.com/
As you can see, these can run into some serious money -- and you still have to check for XP drivers, which may not exist. If you are interested in doing this, be sure to email the shop and make sure the laptop parts have XP drivers. They probably won't be.
"Talking to the desktop" shouldn't be an issue as long as the networking/wireless is set up right since your wireless router should mediate everything. Of course you'll need XP drivers for the wireless card as well as the video card and you'll have to enable sharing on the two computers if you want to share files between them.
As for a laptop card, an Nvidia Geforce GT 650M or above would be fine for adventure games. You can read more about the 650M laptop graphics herehttp://www.notebookcheck.net/NVIDIA-GeForce-GT-650M.71887.0.html