when price is not an issue, do you mean you will spend anything, or the range of 80-100 or so (give or take?)
personally, i was going to go with the 5300's about a month ago, but decided against it.
i have heard that the altec lansing 251's were a bit better than the z-640's, but can't remember where anymore. i'm sure it's just off of one of many review sites i visit....but i don't know the features of either, and maybe that's a big selling point to you.
actually, when i get a new computer (soon, i hope), i plan on buying a receiver and speakers to go with it instead of computer speakers. you can hook the computer to the receiver no problem if your sound card supports it, and a receiver can handle a DVD player, VCR, cable, CD player, you name it.
of course, it'll cost a bit more, but the functionality beats any computer speakers I've seen hands down.
-sandlizard