Anything below the HD 5670 (in the HD 5xxx series) is considered an entry level / multimedia card. The HD 5670 is considered lower-end mainstream and has decent performance for resolution up to 1680 x 1050 with moderate graphics quality, generally speaking.
Using DX 11 can place a significant load on the GPU so that can choke performance.
4870 cards are DX10/9 only. 5670 on paper might appear to have good frame rates for gaming but actually fails at doing so........... maybe 1024x768 and low to med settings. Bad advice. Combining a 4 series with a 5or6 series probably wouldn't work. Running DX11 will put a strain on higher end cards..... moot.