The only difference between the different branded GFX cards is: cooling solution, special offers (like eVGA step up option) and sometimes some come bundeled with gaes. Basically, the card itself is the same. Out of those, I would go with the GT- more performance in games.
Also, before you make the upgrade, I would advise you to check out your PSU- not only the Watts but also the Amps- you may need to upgrade that aswell.
EDIT: If you want to have much more performance, for only a little boost in price, I would reccomend this:
OcUK ATI Radeon X1950 Pro 256MB GDDR3 HDTV/DVI (PCI-Express) - Retail
Though it may be loud, and I would also reccomend that you DEFINATELY check your PSU before buying that, as it is fairly power-demanding.