OWC 6Gig Memory in Mac Book Pro

I've installed the OWC 4 Gig memory into my MacBook Pro taking it up to 6 Gig but it doesn't feel like 6 Gig. Istat always reports about 1.86 gig of inactive memory regardless of how many applications I open and it always starts to get sluggish after what I think is about 4 gig worth of apps and images - but this might be a bit physcosymatic with Istat reporting all the 'inactive' memory.

Do you think there is something wrong with my memory - is there any way I can test it?

Thanks