New graphics card, Old mobo

So...

i currently have an old motherboard, i think it's 3 Years old that i got when i bought a HP Elite computer and the manufacturer is Pegatron or cleveland.. something like that and i am wondering if i should buy a new graphics card like an GTX 670 or 680 but i'm not quite sure if it would work or make a serious bottleneck.
and also i do have an intel i5 2500k, geforce gt420, 4GB ram 1333Mhz 1 TB HDD.

i am really confused, because i'm always thinking if i should buy a new motherboard and cpu, or just go with the same mobo same ram and same cpu and get the new Gpu.

also, the case that i am going to use is corsair obsidian 550D

please help !!