You can pay anywhere from about $1000 to about $400 for adjustable upper rear control arms for the 996. I paid a little over $400 for a set of four from a seller on e-Bay. However, I do not see him posting anything now.
You can buy a set of four Agency brand arms for $625 now on e-Bay (
Agency Power Porsche 996 TT & Carrera Rear Control Arms - eBay (item 110660156263 end time Apr-09-11 23:54:05 PDT)). They look the same as the ones I have, but mine are not anodized blue, just natural metallic color.
Good luck in what you do, but I would not just put on the H&R springs without taking into consideration the camber problem and the effect it will have on your tire wear.