How to know if the GPU is good?

Hello im teh Q
Im new to mining, but after a day or so i make my pc fulltime mine machine :P
doing about 300 KHash/s with nividia card and cpu

But now want to buy a new pc for mining but the gpu?
How do i know that the GPU that i want to buy is good for mining?

Im thinking about 1-4 times a Radeon R9 280X
I see a lot of people taking about the Radeon HD 7970
So after a [url=http://www.hwcompare.com/15861/radeon-hd-7970-vs-radeon-r9-280x/]compare[/url] it looks like im going the right way, but the intire pc is about 1200 â‚¬ for me.
How can i be sure i get good mining Khash/s ?

I hope someone can help me out with this so i can order this pc or need to find a new setup.
Thank you for any replys

ChekaZ

Welcome ImtehQ!

If you like you can add yourself to our FeathercoinMap: [url=http://local.feathercoin.com]local.feathercoin.com[/url]

Yes a saw the [url=http://local.feathercoin.com]FeathercoinMap[/url] before i posted, but as i have a partner i can not just add myself just yet.

@[url=http://forum.feathercoin.com/index.php?action=profile;u=18820]midwaycy6[/url], i dont understand that table, can you explane ?

-Added 5 minutes later,
I understand the table now, thanks.

Thanks,
Still need to find a good rig to build.
Seen some wood rigs but i prefer en case.

I added a single Sapphire R9 280X Dual myself and its going@750kh/s with these settings

–gpu-engine 1070 --gpu-memclock 1500 -I 13 -w 256 -g 2 --thread-concurrency 8192

as soon as I go higher on either core or memory, hashrate drops/bounces.

Thank you for that information. :D
Now i can recalculate my stuff.