The version of Metal Gear Solid: Peace Walker in Metal Gear Solid HD Collection has a bunch of upgrades, most notable amongst which is the fact that it runs at 60 frames-per-second, as opposed to 30fps, which is what the original PSP version ran at.
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Another upgrade, Kojima reveals, is that Peace Walker will support 4-player online co-op over Xbox Live and PlayStation Network. The original PSP game only had support for local cooperative play. Konami tweeted the above image of Kojima giving the co-op feature a go.