Early 2011 MacBook Pro 17" to 30" cinema display
Your particular model of MacBook Pro was equipped with an AMD Radeon HD 6750M dedicated graphics card and on-board Intel HD 3000 graphics card. In terms of VRAM you have 1 GB available on the AMD and 384 MB on the Intel (shared with main memory).

This combination is capable of up to 2560 x 1600 pixels on external displays that do not require a dual-link DVI connection.

If you're finding that the highest available resolution is 1280 x 800 instead of 2560 x 1600, this is likely due to you using a single-link DVI connection on a display that requires a dual-link DVI connection. Unfortunately, the 30" Apple Cinema Display is one such monitor.

You would need to connect this display to a dual-link DVI port to access the higher resolutions available on the 30" display. However, your model MacBook Pro does not provide a dual-link DVI connection without an appropriate adaptor and that is why it's not supporting the 30" Apple Cinema Display at its native 2560 x 1600 resolution.  

For best results you will need to use a Mini DisplayPort to Dual-Link DVI adapter.

You should refer to this info on Apple's website. Also of interest may be this Mini DisplayPort adapter FAQs page.