Firstly don't even bother with that card this one will blow it out of the water
Newegg.com - POWERCOLOR AX4870 512MD5 Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
or a 4850 if you want to keep it under that budget
The chip used obviously one is more powerful then another
Extras such as games, different types of coolers, speeds of memory, speeds of core chip, warranty lengths and target market + rebates throw everything back into the mix
Essentially the resellers buy there chips from nvidia they stick a label on the stock cooler and ship it off. Some companies may invest in a better cooler or design there own this takes extra R&D so cost needs to be past onto consumer. Some companies like XFX black editions are faster chips that have been singled out by nvidia and sold as such. Not every chip produced has the same speed, within batches there is normally an average speed that the chip can overclock to but every chip is unique.
Look at the 5 cheapest normally within $10 of each other after rebate. Free games - check the ebay price for the game incase you want to sell on or to see how much it would cost you to get if not free. Warranty- will you keep the card for long and reuse it or quick buy and sell keeping yourself on the cutting edge, is the warranty transferrable, does the warranty cover overclocking. Do you want to overclock yourself its easy to do but you may not want to risk it for whatever reason.