Suggestions/opinions on computer parts

It's high time I get myself a new computer. A friend of mine compiled an initial suggestion list of parts to get, but he's selected some high grade stuff that I'm now trying to compromise on in order to get the total price a bit lower. I already switched the HDD and such and now pondering on graphics cards.

My friend selected an "Asus Radeon HD 3870" for PCI-E with 512MB 256-bit DDR4.

The damn thing costs quite a bit (at least here), so how much better is it compared to, say, the much cheaper "Sapphire Radeon HD 3650" for PCI-E with 512MB GDDR3?

Or rather, is the 3650 one that much inferior that I shouldn't get it if I'm not quite that concerned with getting an über powerful card? I mean I do plan on playing PC games, but I don't need top of the line performance if it'll cost me an extra few hundred euros. Do the DDR4 and GDDR3 make a big difference?



EDIT:

Also, cases. My friend selected an Antec P182 Gun Metal Black with an Antec EA 650-EC EarthWatts 650W ATX powersource. The P182 case is really nice and the compartment system for the powersource and harddrives is pretty good, I'm told. But is it all that much more quiet, less straining on the components, effective, useful and all that compared to something simpler like Codegen's M-610 ATX case with an 400W powersource? I play with headphones, so running noise is not that big an issue, I just want the case not to strain the components with heat.

How likely am I to actually need 650W or is 400W usually sufficient?

Jam it back in, in the dark.

That calculator is pretty neat, thanks xiao. 400W should be fine for my needs.

As for the cards, Nvidia's sets manage to confuse me worse than ATi's. There's a dozen manufacturers for apparently the same model, but the price differences are huge. Is Asus considered a cheapo card maker that shouldn't be bought (concerning the 8600GT 512M for instance?)

There's nowhere I can't reach.