i've never used ATi, so I cant say anything other then there new R770 cards are supposed to be super-powerful, and also you can crossfire cards of diff models, somethin you cant do with nvidia
and for the 9550, it has the same specs as the 9450, just an increased clock speed, it'd be easier to just buy a 9450 and overclock it to 3.0 if u got enough headroom
the tuniq tower is a good one, but i recommend looking at the thermalright ultra extreme-120, a 9450 on stock settings with a scythe kaze usually runs at 34C
the antec 1200 is a good case, but u better be ok with that much blue light, only the top 200mm leds can be turned off, but overall its a good case with good airflow, just remove the filters on the fans, they increase noise levels and also the dust clogs up the filter making less and less air flow in, dust coming in will just settle at the bottom of the case anyways, as long as u dust ur pc off every month or so, you're fine
just for a look, herse what im running
Antec 1200 + Antec Quattro 1000w
Intel Q9450 (TRUE + Scythe Ultra Kaze 3000), mushkin DDR2 1066 (2x2Gb)
EVGA 750i FTW, EVGA GTX 280 FTW, WD RaptorX 150Gb, Seagate 500Gb
Auzentech 7.1 Prelude Sound Card (Altec Lansing 2.1 Surround + Sony Noise Cancelling Headphones)
as you do not have a sound card listed dont worry about it, but if u want to have high clarity sound, u can get an audio card, but if u do, u have to get speakers/headphones of equal quality for the full benefit, good speakers will run around 200-300 while good headphones go around 100-150
as u can see, theres plenty of space inside, and also a slot behind the mobo to tuck in the cables, i was just lazy, i tucked in a couple cords then got tired
just to open another route for you since you're deciding for air-cooling, buy a lapping kit which will run at most 20, basically its just wet/dry sandpaper @ 400/800/1200/2000 grit
you can pick these up at any hardware store, all it does is provides a smooth, even surface after you sand both the heatsink and the cpu ihs, allowing for more contact which will aloow a greater thermal flow from cpu to heatsink, this can lower temps from 3-10 degrees celsius