Trying to build an engineering / gaming machine for $900

drop the 4870 for two 4770's and swap the PSU for a corsair 650tx.

We've had some other threads regarding engineering, video editing, AutoCad, and other professional software. I do have some reference material that might provide you with useful information to help you make a decision. Unfortunately I am not at home right now. I will post the link to the info this evening.

In the meantime, Rosewill power supplies get bashed quited a bit at this forum. Some of their psu's are not very good. It just depends on the manufacturer of a given model and components installed in the psu.

Here's a link to a recent article published at Toms hardware about using two ATI HD Radeon 4770 video cards in dual Crossfire mode which cost just a little bit more than a single 4870:

http://www.tomshardware.com/review [...] ,2288.html

From what I remember your emphasis should probably be on raw computing power.

Here's a link to a recent thread that is somewhat related to your situation:

http://www.tomshardware.com/forum/ [...] e#t1934986

Here are two excerpts from another thread:

I had the opportunity to speak with the design engineers this morning. They are using several different versions of Intel quad core cpu's, 16GB of memory, Windows Vista 64 bit operating system, and AutoCad 2008 which works well with a 64 bit operating system. There are different versions of AutoCad. The older versions do not work well in a 64 bit environment. The newer versions improve rendering and other features. They could not comment about AMD Phenom II simply because it is brand new and was not available when they did their upgrade last year. There appears to another issue - video cards. The engineers felt priority should be assigned to the cpu and lots of memory at the expense of a video card. Their line of thinking was that intensive 3D video games are gpu intensive while the work they do is cpu and memory intensive.

and

For those who actually have a profession of rendering, encoding and the likes, Core i7 usually shows 20~30% improvement over its competitions. Given the fact that it was originally designed as a server oriented CPU, it really justifies the cost when you run CPU-intensive programs (HPC for instance), as it was shown eating two Shanghai alive with rooms in the mouth to spare.

But of course, for someone who thinks "gaming" is the real market, there's not much to argue about.