I am wishing to upgrade my DG31PR with Quad Core 2 GB DDR2 and am looking for suitable entry level graphic card which I'd be able to double sometimes - I know nothing about this - is there a modular graphic card system if you know what I mean (like I buy 256MB card in one slot and then buy another 512 MB after few months to increase performance)
There is no such thing as a modular GPU system. There are multi-card solutions, but you need a motherboard that has two PCIe 2.0 slots, and if this is your board, you don't even have one. No current GPU would be able to run on your board.
I would highly recommend looking at a new build. The Core2 CPUs are very old tech, as is DDR2. If you had about $500 (possibly less depending on your needs), you could build a new system that is relatively future proof.
I would take a look at the guidelines (link's in my signature). If you provide us with that information, we can put to gether a build that will fit (almost) any budget.