Hi David,
just be careful with the Vue 7 xStream purchase. I´ve purchased the upgrade last december (from Infinite 6 to xStream 7 with Max 2009). The e-on marketing said that rendering times are “up to 20% faster” and here at CGtalk there was a test that confirmed that one image took much less time to render in version 7 compared to version 6. My experience was the absolute opposite! I´ve rendered my own scene in Infinite 6 with 10 min a frame. I bought xStream 7 to improve render times but it turned out that xStream rendered MUCH SLOWER, meaning that the same image rendered in 1 hour, not 10 minutes. So this was a slow down and not a speed up. False marketing from e-on IMO!
Second thing to consider with Renderfarms. E-on does provide Hypervue and license for 5 render cows, but ONLY in standalone mode. If you want to use xStream in Max or any other host software, you will not be able to use the rendercows, but will have to by the license server ($500) plus a render node per CPU for each $150. In my case this would have been an additional $1000 investment for a software that is supposed to work “seamless” within 3Ds Max. IMO this is again a total rip off by e-on trying to milk money from their customers.
Third, the global illumination / final gather technology integrated with mental ray does not really work. Shadows are not rendered correctly and lighting is messed up. Integrating just trees in a max scene without the athmosphere will result in wrong colors. Nothing that can be fixed, but very tedious to be corrected by hand.
And still, even with version 7.21, the integration is not really working and xStream is crashing Max very often plus destroying scenes while saving.
Sorry if this is appears to be a rant, but I feel being ripped off. Please just understand this as a warning before spending a lot of money.
Lets hope that the announced free update to version 7.4 (“out soon”) will solve these issues, so we finally get what we paid for.
Other than these problems, Vue has by far the best technology for clouds, athmospheres, eco-systems, and solig growth plants. 
Best,
Martin