I have 56K (bought at 3400mi) on one IS, and 36K on the other. New 440's on both, neither ever rebuilt.
Remember, they are warranted for life.... which they should be for the price. Just keep proof of purchase/receipt. If one ever does fail they rebuild/replace it for free. (40-50K minimum before any trouble/leaking would be my guess... not counting hitting a foot deep pot hole or some such trauma to the shocks). Get 13" and be sure to get the right spring rate for your bike & weights of rider/passengers.
Cheapest I could find anywhere was MAW, but those arseholes charge a surcharge in addition to shipping and handling.... ended up only saving about $40 from HDL. If I was to do it again, I'd get HDL to price match MAW. Also, black 440's will save you about 30-40 from chrome.
A little more info here:
http://forums.delphiforums.com/valkyrie/messages?msg=13294.1