5 DDR3 << 3 DDR5
already posted links about DDR5 vs DDR3 ram. It's a wrap for you nikkaz.
That link you posted said that half a half a gig of ddr5 outperformed 1 gig of ddr3 by 8%. In this case we are comparing 5 or more gigs to 3 also the rumors state that Microsoft has added some extra technology to deal with the limitations of the slower ram meaning that 8% performace increase is negligable, and actualy the Xbox's ram situation is much better than ps3.