Upgrading HD4870

Hello,

I am planning to update my graphic card , that is HD 4870 512Mb for something better , but still am aware of possible situation, that might happen , if i buy some better card , that my current system will not handle , and result ll be just waste of money/ power of the card ll be unused until next HW upgrade.

And therefore I 'd like to ask what card d be still best choice for my current system , trying to avoid the sitatution like Cpu "slowing " the card down , MB is not good enough for this or that type of card etc.

My current system looks like MB Gigabyte EP45-UD3LR with Intel Core 2 Quad 9400 at 2.66Ghz ,PSU is Fortron SAGA II 500W and 4Gb of RAM, but planning to upgrade to 6 or 8 gb maybe at this time.

I know i should upgrade MB and CPU first , but still my priority is card now.
I was thinking about some cards from Nvidia or Ati , but no idea which 'd perform better on this type of system.
Price range for card up to 450 Euro, but really not sure if its worth putting such card into such crappy system.

Thanks for suggestions.