In terms of what manu to go for (eg ATI, Sapphire etc) it used to make a lot of difference esp in the 8500 days where some manu's clock speeds were around 230/333 instead of 275/550. You still get a variance today but that's mostly on the budget cards like the GF4MX, GF-FX5200 and Radeons below 9500. So long as you're shopping above the budget lines you should find all manu's offerings based on the same card are for all intents and purposes identical. You may find packaging, colour, HSF design, bundle and warranty vary so it can be worth going by more than just price depending upon your prefs and reqs. The only manu I would steer clear of is Powercolor as I've simply heard too many instances of their sub-par components esp using 64bit DDR in place of 128bit DDR thus halving the all important RAM bandwidth.
As Mem says, we need specifics to answer you specificly but currently if you have a GF4TI, Radeon9500 or better then it's best to wait as you have to fork out a lot of money for something genuinely better. If you want something now and you don't care too much for AA+AF or DX9 the GF4TI4200 is VERY good value otherwise look to the Rad9600PRO or GF-FX5600ultra (pref v2 flipchip) but expect to pay 50% more cash.