Don't get the 2GB version- it really doesn't give much, if any, improvement and costs WAAYY more than the 1GB. As far as which is a good brand, there won't be much difference between any of the good ones. Sapphire has a good reputation, XFX does as well. The only thing I know of that would really differentiate any of them in general is that XFX has a double lifetime warranty, which means you get a lifetime warranty, AND the person you sell/give it to later ALSO gets a lifetime warranty. Other than that, its really personal preference.
Well, Visiontek and XFX are the ones that come with a lifetime warranty, so that's what does it for me.
I don't know what you'd be doing with a 2GB graphics card unless you have a HUGE monitor and want to play everything at ultra-high settings. For most normal setups, 1GB is plenty, so something to keep in mind. There's a pretty good article about different levels of video card RAM here:
Do not bother with the 2Gb card, it's wasted money.
As for 'best' it depends on your country.
In the US/Canada XFX have the double lifetime warranty but BFG and EVGA also have lifetime warranties. These three would be my best choices with one condition from personal experience: Avoid XFX cards like this:
The cooler is just hopeless: Noisy and hot.
As for powersupply, PC Power and Cooling, Antec, OCZ, Corsair, Seasonic and Silverstone are amongst the best.
Make sure whichever unit you purchase has two 6 pin PCI-E connectors ( required for the HD4870) and about 550-600Watts output, more might be nice but might be wasted money.
By the way, what are the rest of your system specs? If it's a new i7 machine, you may consider a 5xxx series card so you can get DirectX 11. Your system will last you longer without having to upgrade again.
If it's a Core 2 or quad core, a 4870 or 4890 is an excellent choice.
Microsoft® Windows Vista™ Home Premium , Service Pack 2 (64xbit)
AMD Phenom(tm) 9650 Quad-Core Processor
WDC WD6400AAKS-22A7B0 ATA Device, 596.17 GB
PIONEER BD-ROM BDC-202 ATA Device
KPGB OHMNO1Y3ST SCSI CdRom Device
ATI Radeon HD 3200 Graphics
ATI Radeon HD 3400 Series (256 MB)
Total Available Graphics Memory
ATI HDMI Audio
ATI HDMI Audio
Realtek High Definition Audio
Marvell Yukon 88E8071 PCI-E Gigabit Ethernet Controller
D-Link WUA-2340 USB Adapter
BIOS Release Date