Also what games you want to play, what settings, what monitor resolution, what budget.
To get started, you might want to take a look at upgradevideocards.com. In addition to a lot of information about video cards, they have a page about defining what you want and need in a system - your special features and system requirements.
we need more info than that mate, games don't solemnly run based on your graphic card.
CPU speed ? - and how many cores
RAM ? how much, and what frequency?
Hard drives (with focus on the RPM)
and like Rockyjohn said, what games, is it pacman or crysis 2?
what budget do you have? 10$ 500$ ? "not too expensive" is based on where you're from
also, most graphic cards nowadays (the good ones) needs 6pin-8pin power plugins (or mine that need 2x6pins each), so you might want to check if your power supply has that as well.
CPU speed 2.5 GHz,Cores:2
RAM:4 gb ddr2
DRAM frequency: 399 MHz
well games like cs to have fps 100,idk which because i didnt have the chance to play more games...
about 100$ i will change my power supply,thats not a big problem
If you need a card that costs half of what the GS 450 costs, you are essentially talking about non-gaming video cards. I suggest you look at the on the page for non-gaming video cards at the site I linked above. It has makes and models at different price levels.