I've had a similar problem. I've been running my FCP suite on a quad until I bought an 8Core last month. When I opened a job I onlined on the 4Core (with the ATI1900 card) on my new 8Core I found one or two shots with filters applied (blur) that would not render. I have deducted that the ATI HD 2600's 256mb of video memory is not enough to render the filter which did render with the ATI 1900's 512Mb of video memory.
I'm still looking for a way to upgrade my 8Core without having to resolve to buying a new ATI1900.
Llewelyn @ Fireworks
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http://www.fireworksfilms.co.za