Which nVidia graphic card can my desktop support?

It’s pretty old. Here are the specs:

processor -> intel pentium e5300
motherboard -> msi boston 1.0
windows 7 32 bit OS
2gb ddr2 ram

As long as your graphics card is not Integrated into the motherboard then ur okay!
Nvidia Quadro series is good

As your CPU is not close to top, you maybe stay in “GTX 460” range cards, any lower will give you almost zero benefit against CPU if you will try to use Blender Cycles in GPU mode, any higher then 770 will be too fast for CPU/RAM and GPU maybe will wait data from CPU. FYI, wait a week at last, as one of GPU maker (AMD) will expose next generation card and prices can change a lot. Try to get NVidia card wit more onboard RAM if it an optiion, as Cycles render must fit all scene related data in it or you get black picture or even crash. 1 GB is very minimum. But it will work in CPU only mode, ofcourse slower.

If you dont mind Blender Cycles in GPU mode, then get any card that fit in PCIEx17 slot and in case, it will work ok if your power adapter is enough (top cards need additional 300Watts, middle 150, etc)

GTX cards are good but it’s more for video games. Quadro cards are designed for 3d graphic design!
It comes down to what you can afford. Your motherboard might slow everything down though…

@CPU, an expensive proffesional NV Quadro card for a midrange cpu from 2009 on a lowend motherboard?? This would be a waste of money!

I would buy a cheap Nvidea GTX card like storm_st mentioned, every Card over 150$ is a waste of Money for your System.

A Quadro card for Blender is a massive waste of money no matter the mainboard.
Blender does not really use Quadro features and a Quadro is much slower in Cycles than a GTX with a compareable GPU.

Please stop this nonsense. Quadro is absolutely useless for Blender. Anyone considering a GPU for Blender Cycles needs to look at gamer cards. They are simply faster and for a good price compared to that Quadro parody. For reasons why it is how it is, read this - http://www.systemagnostic.com/faqs/

If you want a “professional” grade GPU, buy Tesla - it`s actually useful for Cycles. Geforce Titan though provides comparable rendering speeds for half (if not even less) the price of Tesla.

EDIT: @OP, your motherboard can support any of the newest GPUs, it all comes down to your budget (and case size, high-end GPUs can be huge). Also your other hardware parts wont slow down Cycles rendering too much as most of the work is done in GPU only. Even PCI-E slot speeds don`t matter much, this is not gaming.