PowerPoint won't do EPS and it's improbable that they really can accept PS
(assuming that means PostScript, which it may not).  

PowerPoint will save as TIF, though in some versions, XP is one, I think, the
TIFs it saves are sometimes corrupted.

Better to save as PNG then convert to TIF.  The publication will never know the
difference, as there will be none.

But what does 600 dpi mean?  DPI = Dots Per Inch.  A ratio, not an absolute
measurement.  If you know, even roughly, how big they plan to run the diagrams,
you can multiply that by 600 or 400 and work out the needed size in pixels.

Pixels we can conjure with.

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
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What Steve said.

What I would do is save the PPT slides as WMF or EMF, open those in
Illustrator, and then save as EPS. I'd also convert the text to outlines
before saving the EPS.

When you save as EPS in Illo, you won't get an option to set PPI (DPI)
because EPS is vector and it doesn't matter. If your slides are text,
charts, tables, clipart, and/or drawn autoshapes and lines, I'd do EPS. If
you have images on your slides, I'd go the TIF route. If you export (use
File-->Export in Illo) as an image type (PNG, TIF, whatever), you'll have a
chance to set the PPI (DPI).