Hell with no budget get this bad boy
ASUS Dual GeForce 7800GT EN7800GT DUAL/2DHTV/512M Dual Core Video Card - Retail
$812.00
In the high-end you've got the nVidia 7800GTX and ATi 1900XTX series cards.
More realistically a 7800GT Extreme is a very good card for the price
Leadtek GeForce 7800GT WinFast PX7800GT Extreme TDH Video Card - Retail
$294.00 this is the card I personally use.
In the $200.00 range are cards like the 7600GT and ATi x800.
Below $200.00 there are a lot of different cards with different specs and advantages, so if you want something in that range let me know.
(All these prices are being pulled from NewEgg.com)
Another thing you'll need to consider when upgrading cards, is if your existing power supply can handle a new high end card. I'll need to look up the PS in your eMachine. I don't think it will, since right now you're using onboard graphics, which means that basically top-end cards (200.00 and up) may be out of the question.