Do you currently have only 1x 2GB module installed? If that's the case, then try to get the same 2GB module, so that you can keep your existing motherboard. There's really no need for 8GB of RAM if you've managed with 2GB up till now!
Also, you want to run the memory in pairs of exactly the same modules (same size, timings and clock speed), otherwise you'll lose some performance (~15% memory performance, but not as much overall performance) due to it not running in dual channel mode.
Both the GTX 460 & HD 5850 requires 2x 6-pin PCI-E power connections, however they usually (if not always) do come with 1x Molex (4-pin) to 1x 6-pin adapters so that you can use it along with a 500W+ PSU that has only 1 connector.
It's really difficult to choose between the GTX 460 & HD 5850, as well as between the difference models and brands.
It all just comes down to what you're willing to spend (please note that these price ranges are just estimates):
- GTX 460 1GB @ R1800 - R2300
- GTX 460 OC 1GB @ R2300 - R2800
- GTX 460 SOC 1GB @ R2800 - R3300
- HD 5850 1GB @ R2700 - R3500
- HD 5850 OC 1GB @ R3000 - R3900
Don't be fooled by marketing stuff with the GTX 460 models, where they'll say that the specific card has "Military Class" components.
The GTX 460 reference models comes standard with "Military Class" components

and they can be overclocked reasonably well too, but the heatsink & cooler then comes into play!
Before buying a specific model, make sure that you read reviews of it first.
Currently I don't know which one to choose:
- 1x MSI GTX 460 Cyclone OC 1GB @ R2507
- 1x Gigabyte GTX 460 OC 1GB @ R2633
- 1x Club3D HD 5850 1GB @ R2758
- 2x Inno3D GTX 460 1GB @ R2068 each
All of the cards above use non-reference coolers, with the Inno3D GTX 460 1GB model's one being the poorest of them all since its very close to stock cooling, unlike the other cards with their high-end coolers.
Now I'm just waiting till 22 Oct...