Get GPU-Z. It will tell you exactly how much memory on your graphic card is in use.http://www.techpowerup.com/downloads/SysInfo/GPU-Z/
You can also optimize your PC and Poser. With GPU-Z you can see how much you normal start-up windows version is using. Mine was using 128MB before but is down to 47mb now by turning off some of the windows graphics options.
In Poser under Render Settings/Preview/Texture Display you'll find Preview Texture Resolution. Keep that as low as possible as that will eat up your graphic RAM if it's set too high. Again watch GPU-Z to find a good solution for you.
Do not have other open programs running in the background. They also need RAM from you graphic card to run. I know Vue can easily eat up 1GB of RAM.
If you can, keep you textures to a minimum. Delete those you do not use anyway, or reuse some on other props to keep the texture number down and also the amount of RAM used.
Reusing textures does not use more RAM!
High resolution images take up a lot of RAM on the graphic card. If you run out of RAM, you can resize the textures to a smaller resolution to free up some RAM. It's very effective! In theory you can just keep downsizing the textures until they fit in your memory.
Nvidia Fermi cards can only load 64 LDR RGBA textures. The 600 series (Kepler) can in theory load 256, but is software? locked at 144 at the moment (144 LDR RGBA, 68 LDR greyscale, 10 HDR RGBA, 10 HDR greyscale). This might change when Octane moves to Cuda 5.0
If you have a graphic card of 2GB of RAM and buy one more with 2GB, you do NOT get 4GB of RAM to play with. The RAM on the cards can NOT be shared with other cards.
If you have multiple Nvidia cards with different RAM sizes and want them all to render, your RAM is limited to the card with the lowest amount of RAM. So if you have a card with 4GB and one with 1GB, you can only use 1GB if you want them both to render your scene.