It will be compatible, the ram has nothing to do with eachother as the ddr5 on the gpu is used for the gpu and the ram on your motherboard is used for the cpu. GPUs need fast ram which is why they have them.
I'd suggest against the 550 ti because for its price you can generally get a 6850 for better performance or get a 6770 for cheaper for the same performance as the 550 ti. Its only good if you need cuda or physx.
bcoz I've already bought the 550 ti. I'll give it a crack for a few months until I have enough to get enoughcash for t he 6850. I checked reviews on it & it seems well worth having.
Would you suggest I keeping my P5G41T-MLX board when I upgrade the processor to Core i7 930. Or would the processor's performance be more sufficient with another board? can you suggest other boards that will boost performance.