Well, yes... That sort of makes sense. But let me explain why that is. Basically, if your memory is low say 512 MB for instance.. You are far more likely to encounter paging slowdown. If the system runs out of memory your pc will engage virtual memory. This is extremely slow compared to direct ram access. For instance, that 1024x1024 texture that would normally run really fast will suddenly chug like your pc has caught a serious disease. This is because instead of writing to your ram, it writes to the hard drive.
Memory is so much faster than your hard drive it is hard to really explain. For instance if you load up an object off the hard drive and compare that to when you load it from memory, say a memblock for arguments sake, you will obviously see a major difference in speed. When you call an object up in DB you are basically loading it from memory. It is instant, when compared to loading from hard disk. I am not explaining this very well, but I am sure you get the idea.
Of course the video card is vital too, in fact almost more so. BUT, if you have low memory it will always restrict your video card. It is no good having a Geforce 7950 512MB that has cost you a small fortune, if you have 256MB of ram. It is a balance. I have 1.5 gig of ram which is more than needed most of the time. This allows my 7800GT to run at pretty much full whack (my processor will still be holding it back I would think), so I have no problems running any game. If I took out 1 gig of that memory my system would be crippled in comparison, because windows would have to page memory to the hard drive instead of straight from memory. So even thought my video card is very fast, without a reasonable amount of memory to be had, it is a waste of time.
Heh, that was a long rant. I'm sure that covers it, sort of. Anyway low memory is bad. High is good. But without decent hardware to support it, there is a limit to what more memory will do for you.
Hope that helps explain things..