I've got:
-a new laptop with Vista and an old one with XP
-recovery disks for both laptops
-all downloaded items from my XP laptop saved on a 500GB external HDD

What would be the easiest way to get XP + files from my old laptop onto the new one, without physically switching the HDDs? I was thinking of erasing the new HDD, using the recovery disk to get XP on it, and then transfer the rest of the files onto the new hard drive via my external HDD. the only problem with this is that I don't know how to erase a hard drive, so unless someone has a better idea, just tell me how to do that.

or if you think I should just keep vista, post arguments for that

If you carry your PC around a lot it might be wiser to keep Vista (with classic interface for battery++) since it's more secure (in case of theft). It also has much better mobility settings than XP.

I second keeping Vista. There's a good chance your laptop manufacturer will not provide you drivers necessary for it to run XP, or at least you will go through hours of frustration trying to find these drivers. When I tried to put XP on a Vista laptop, I was able to get all necessary drivers except for the network drivers, which didn't exist for XP. With such a critical piece of the driver puzzle missing, I had to do another format and put Vista back on.

Keep Vista, buy more RAM. I dunno about where you are, but in the UK laptop memory is cheap as chips

Render,

I unfortunately have to agree with most people here.  Vista is something you probably need to keep, because most manufacturer's will not give you drivers/etc for your laptop to run with Windows XP.

Also, you can't use the recovery CD from one laptop on another.  Unless, you're incredibly lucky or have a Dell, which I believe will work on all Dell's.  But that may only be an install disc -- I don't know about a "recovery" disc.

--Terin

Most the time, moving an install of XP from an old system will cause nothing but problems (if it even starts up). If you're that adamant about getting XP on it, check if your manufacturer offers a downgrade. I've put XP on a lot of newer laptops, and haven't had any issues. Although there is a possibility to run into something like Mr. Epic Hero said, it's honestly quite rare. Lack of drivers is more of a concern when going from XP to Vista than vice versa. If you do decide to reinstall XP, you'll need the install CD for sure