4870 512mb (old) + new direct x11

Sapphire ATI Radeon HD 5450 512MB GDDR2 ATI Radeon HD 5450 Chipset (650Mhz) 512MB GDDR2 (800Mhz) Dual-Link DVI/HDMI/D-Sub PCI-Express 2.0 Graphics Card

Basically I see it as dx11 only, is it worth it to play games like aliens vs predators and other dx11 games high resolution? (never been a problem for me playing on highest performace)

You cannot xfire any dx11 cards with the 4870.

Buying a weaker card with dx11 is pointless because most of the time you won't be able to produce playable frame rates in dx11 mode anyways.

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.