While you can get madVR working on lower-end cards like a GT630 - I've had it running on a GeForce 9400 and a GT610 - if you're buying a new GPU for it, I would not recommend anything less than a GTX 750. (and I'd recommend a 2GB card if you can afford it)
Using a lower-end card with madVR is only going to let you use the basic scalers, rather than the more advanced ones.
Those are still worthwhile - there are more reasons to use madVR than just the scaling algorithms, but if I were buying a GPU for it, I'd want something that can handle some of the more advanced options.
madVR is not a "standard" video renderer - it uses the GPU shaders to perform advanced scaling and processing, which is why the requirements can be so high.
I don't think even a GeForce Titan Black, or an R9 290X can handle it at the highest quality settings - but there are settings which give you 90% of that image quality at a fraction of the performance requirements.