According to the reviews both of the coolers I listed come with thermal paste which I'm sure is adequate for the job.
If you have your heart set now on a HD 6870 you could order a cooler and the HD 6870 then send the HD 6850 back under the Distance Selling Regulations for a refund.
All you'd lose would be a few quid postage to send the card back.
If you have your heart set now on a HD 6870 you could order a cooler and the HD 6870 then send the HD 6850 back under the Distance Selling Regulations for a refund.
All you'd lose would be a few quid postage to send the card back.