Ok, first off it's interesting that the RAM is socketed. In my experience this is unusual. By the time the Atari 800Xl came along almost all home computers had their RAM soldered in (I need to check my own Atari 800XL...perhaps it's an exception with these ones). Anyway, if it is socketed that's good news for you! It might also indicate the RAM has been repaired before. Has it been repaired properly and were the right ICs used might be a question to ask?
I'm not familar with the 4264 DRAM chip. 4164's are more common for this vintage of machine, but maybe the Atari XL 800 used them. 4264 and 4164 ICs might even be interchangable...I don't know. Sometime today if I get a chance I'll whip the top off my own 800XL and let you know what's there.
Getting a replacement IC might be tricky, especially if you need a 4264. I've used this USA firm in the URL below in the past. They have quite a few RAM chips but not a 4264http://www.arcadecomponents.com/memory.html
. Also, shipping is expensive and takes a while. It's really only viable if you have a number of components you want to get.
A good strategy for a replacement component is to look for old battered and broken unts on trademe that go for a few bucks. These are a good source of components and will probably be cheaper than getting a single IC in from overseas. Desoldering is usually necessary of course. If the case is wrecked sometimes the board is still ok, so you can just substitute it.
Anyway, I'll take a look in my own machine and get back to you. If anyone else wants to contribute into this thread, feel free.