Nothing you do in photography needs a fast, powerful graphics card, as you're just displaying a 2d bitmap on the screen. A fast graphics card for a machine that only deals with photography can be any old piece of crap so long as it has enough VRAM to drive your display.
Lightroom should be perfectly happy with 8GB of RAM BTW.
While Photoshop has some GPU accelerated features, they're pretty basic and any GPU made in the last 5 years, no matter how crap can cope with them.
The only use for powerful GPUs is gaming, CAD/Design, 3D applications (modelling and rendering that uses the GPU for rendering) and any video editing software that uses the GPU for actual preview and rendering (CUDA etc). There are other uses for a fast GPU of course... but photography isn't one of them.This is why Macs get away with having such s**t GPUs
If all you do with your machine is browse the web, office apps, and photography... then most motherboard's onboard graphics is more than enough.
Out of the two options.. GPU or RAM, I'd upgrade my RAM. It will make little difference with Lightroom unless you are working with DNG files created on a IQ180 or something, but Photoshop LOVES RAM once you start working in 16bit with layers.
Your machine should be able to cope with Lightroom easily though. Is t not fast enough or something?
Caveat:
On-board graphics often shares DDR system RAM, so a discreet GPU instead of on-board graphics can yield a slight performance increase.... but it would be cheaper to add RAM than upgrade the GPU.