It is an issue regardless of which card you choose (nvidia or ATI), these cards all need to be fed.
Your friend is sadly a misinformed nvidia supporter (I'm not trying to be rude, but it's simpler and faster if I just say it), and it's probably not his fault, but due to the fact that nvidia's marketing department is a force to be reckoned compared to AMD's poor marketing. I blame AMD for this, not nvidia.
Reason all this happened is because in the past two years ATi failed to deliver on time the R600 chip (HD 2900XT card), and when it finally did it was a big disappointment, it gained some ground, albeit not much, with the RV670 chip (HD 3850 and 3870), but it really shook the world with the RV770 (HD 4850 and 4870).
Basically all problems present on the previous generations (R600), like crazy power consumption, horrible AA performance, and subpar performance have been addressed with the HD 4000 series. You will find that using 4x AA in the HD 4850 and 4870 is basically "free" meaning that you'll take very little if any performance hit, the power consumption is acceptable, and the performance is all around great.
Some people complain about heat issues with these cards, but they fail to realize that they are built on the A12 silicon, which is designed to run hotter than previous cards. But if you're concerned about the little extra heat, you can always get a card with a non-reference cooler for as little as free up to $30 depending on the manufacturer. This is basically what I did with my HD 4850, I ordered the Sapphire Dual Slot edition and for you to get an idea, reviews show reference 4850s running at a load temperature of 85C, but mine doesn't even go above 59-60C under load in Crysis.
Hope that answers your question about the Radeons, they are not inferior in any way to Geforces, in fact this time around, the Radeons are the better cards on most scenarios, leaving only a few viable price points for nvidia's offerings to shine.
Looking at these benchmarks, I'd say you'd be running at 60+ fps most of the time even with the HD 4850, since the HD 3850 is already running at 50fps and it's A LOT weaker than the HD 4850.
PCGH Tuning: Warhammer Online - Age of Reckoning
http://www.pcgameshardware.com/aid,660322/News/PCGH_Tuning_Warhammer_Online_-_Age_of_Reckoning/
What you would have to do to install the graphics card is the same thing you would have to do even if you were to get an nvidia card. First download Driver Cleaner Pro, install it and follow these directions:
1.) Go to Add/Remove programs, uninstall nvidia drivers and restart.
2.) Run driver cleaner pro, set nvidia as a filter. Shut down the PC.
3.) Remove the current card and replace with the new one.
4.) Install latest drivers for the card.
The only special thing you'd have to do is to get the latest Catalyst drivers from ATI's website (8.9 is the current build).
My view on both the HD 4850 and 4870 is that they are terrific cards, incredible AA performance (I hadn't seen cards that performed so well with it enabled), and I'm definitely looking forward for future games that'll make use of their capabilities (tessellation and DX10.1 features). If you have the money to get the 4870 then do it, but you can't go wrong with either.
I suggest this PSU as a bare minimum, regardless of what you plan on doing:
CORSAIR CMPSU-450VX 450W ATX12V V2.2 80 PLUS Certified Active PFC Power Supply - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003
EDIT: About power supplies, it not so much about the wattage, but the 12V amperage rating. You are recommended to have at least 30A on the combined 12V rails (it's not as easy as adding them up to find this number, though) for the latest cards to run properly. The PSU you showed has approximately 30-34A on the 12V rails (Can't give you an exact number, because thermaltake is only disclosing enough info to make an educated guess), but the one I showed you has a certified 34A across the 12V rails, regardless of it being only a 450W unit.
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