I've always been curious as to how my old iBook stands up to Apple's latest and greatest systems to see if it really is worth it to plop down $1000 for a new system. Register Hardware did some Xbench comparisons of the 2006 MacBook Pro 1.83, 2007 MacBook 1.83ghz, the early 2008 1.6ghz MacBook Air, and the 2.4ghz new Macbook for this year. All systems run on Intel's Core 2 Duo processor. My system is a late 2005 1.33ghz PowerPC G4 single core processor iBook G4. I ran my own Xbench results and came up with some interesting results. Xbench gives its results as points.

First off, we'll look at system Spec.
MacBook Air Early 2008: 1.6ghz Core 2 Duo "Merom", 2gb 667mhz DDR2, Intel GMA X3100 integrated graphics with 144mb shared DDR2, 80gb 1.8'' PATA HDD @ 4200rpm

iBook G4 Late 2005: 1.33ghz PowerPC G4 Single Core, 1gb 333mhz DDR1, ATI Radeon 9550 32mb discrete GDDR, 40gb 2.5'' PATA HDD @ 4200rpm

CPU:
MacBook Air = 77. 83
iBook G4 = 62.68
Difference = 24%

Memory:
MacBook Air = 127.09
iBook G4 = 31.75
Difference = 400%

Quartz Graphics:
MacBook Air = 96.97
iBook G4 = 60.06
Difference = 61%

OpenGL Graphics:
MacBook Air = 13.92
iBook G4 = 63.12
Difference = -453%

User Interface Test:
MacBook Air = Unknown/Not Tested
iBook G4 = 16.89
Difference = N/A

Disk Test:
MacBook Air = 20.30
iBook G4 = 33.76
Difference = -66%

Overall:
MacBook Air - 40.65
iBook G4 - 36.90
Difference - 10.2%

I don't know how the Register did their tests and these are synthetic benchmarks, so I can't say these results are definitive. Real world performance with the Air might be better. However, the results are still interesting. The MacBook Air's 2gb of 667mhz DDR2 gives it a huge edge in memory power. It's also significantly faster at Apple's own Quartz rendering. However, in all other options, the Air is not significantly faster than its older cousin. The iBook shows that dedicated graphics can still make a huge difference over Intel's integrated solutions, even though the iBook's GPU is considerably dated has less memory to access. The iBook's hard drive is also significantly faster despite being the same rotation speed. With everything factored in, the Air only works out to be 10% faster than the $999 iBook, which can now be fetched used for well under $500. The Air's base price is $1799. So Apple releases a notebook that's 80% more for only a 10% increase in performance. While the Air might be the thinnest and lightest laptop you can buy, this doesn't exactly scream good value. I am aware the Air does have other additional features such as iSight and 802.11n built in. The current vanilla MacBook at $1299 is 299% faster than the iBook G4 according to Xbench tests.

MacBook Air Benchmark Source: Register Hardware