Sales Page on a different domain than members-only forum?

Although I've been really wanting to get into IM for a while now, this time I'm finally doing it! But I need your advice... It's been said that a continuity program is a good business model, so that is my first venture. Taking a page from copywriting 101, I've adapted the "swipe file" concept to include websites too.

The program I like, and have gotten much of my inspiration from, is another weight loss/fitness product. There's a website on its own domain for the sales letter which sells the customer some downloadable ebooks and access to a members-only forum (which must be manually approved after purchase). All this through Clickbank. So far, I plan on the same, only the example uses a one-time charge for indefinite access to the members-only forum. I will use a $1 charge for the first month and then probably about $37 per month thereafter.

The program I'm partially modeling this after uses a sales letter on its own domain. Prospects are driven to that page from either the site owner's personal blog (on yet another domain name), joint venture or affiliate emails or even from organic search engine results. Lastly, the members-only site is on yet another domain name.

Is this a good idea? Or should the sales letter be on the same domain, but a subdomain (like salesletter.mysite.com) or another directory (mysite.com/salesletter.php)?

My concern with having only one domain for the members-only forum and the sales letter is that the sales letter would be on the home page of the website would be seen by everyone, on every visit, whether or not they purchased the product. This could be annoying for people who already bought and just want to get to the members-only forum. I thought about a link at the top of the home page where it would say "members click here" and that would take them to the forum.

Or would it be better to just one domain name for the sales letter and another domain name for the members-only forum (and downloadable resources)? The only downside I can see to this is building up a good page rank or Alexa ranking would be watered down, as the sales page would likely only be visited up to an including the moment where someone purchases. Then the would go to the other domain name for the forum. I've seen this done both ways, but with different kinds of products. Just not sure which is the best...

What do you think is the best approach for this? Thank you in advance for any and all suggestions.