Crysis is still considered to be the best looking PC game, but I prefer the looks of Killzone 2. It's due to a combination of art direction/assets, amazing effects (such as the game's lighting) and animation.
PC vs PS3, modern PCs have the advantage of being able to do higher resolutions and higher framerates. Having said that 720p with a solid 30 FPS is enough for the bulk of console games to provide a superior experience on a big HDTV screen (vs the 17 inch monitor I have on my desk and I don't want a bigger screen in my workroom anyhow). For me no fuss living room gaming is far more appealing, wireless Sixaxis controllers, big screen HDTV, kick ass 7.1 audio, etc.
The PS3 has some important strongpoints compared to PC game potential, for one developers can focus on one specific core hardware configuration, secondly there is the Cell processor which provides various potential advantages (with effort) over much more costly PC processors regarding games/multi-media potential and the PS3 has Blu-Ray which allows for more content and graphics/audio content on a single disc.
In any case Killzone 2 will be PS3 exclusive, nomatter costly modern PCs are capable of doing the game better in theory the game will not be on the PC. Killzone 2 may well become the best looking FPS. A new PC game may require 6 GBs of RAM, 1 GB video RAM, a 16 core CPU and what not and run in 120 FPS, 2160p and still not look as good as Killzone 2.