Preferably AMD Crossfire and at least 3GB for a reasonable price. I am looking to upgrade my PC a bit and a Graphics card need upgrade the most. Also, will I need to buy a new PSU, I have cx750 at the moment and it works no problem so far
For best matching, you must CF cards with the same or close specifications; in your case, the HD7870 will go with a 2GB R9-270 (270X for the GigaHertz Edition). If you go with an R9-280/280X, you will not be able to use the additional 1 GB of VRAM, and the performance will be lowered to match the 7870 performance, so basically you will have the performance of a HD7870 CF setup with additional tearing/stuttering effects.
For the higher end of your budget, you could get a GTX 970 that would be much better than your 7870.
EDIT: Whoops, I didn't notice your AMD preference. Well you could snatch something like a 290 if you really need to have an AMD card. I don't know why in the world you would want a crossfire setup, so I can't really recommend one. I would still go for the GTX 970 though.
Will 'Inno3D iChiLL GTX 970 HerculeZ X4 Air Boss Ultra graphics card - GF GTX 970 - 4 GB' do a good job and does the card has to have a AMD chipset to work with my processor and motherboard?
Will 'Inno3D iChiLL GTX 970 HerculeZ X4 Air Boss Ultra graphics card - GF GTX 970 - 4 GB' do a good job and does the card has to have a AMD chipset to work with my processor and motherboard?
Sorry for the late reply but anyway yes that card is a bit overkill on the cooling side but yes it should work just fine with the rest of your build.