I wouldn't be so sure:
http://www.dxomark.com/index.php/eng/DxOMark-Sensor
Note that the D300 ranks #25 in Low-Light ISO, behind such cameras as the Pentax K200D, K20D, Canon 20D, 30D, 40D, and XSi. They don't seem to have tested the A700, but these sensor tests suggest that the stellar high-ISO performance of the D300 is something of an urban myth.
When the D300 was released, reviewers were comparing it against the previous generation of DX cameras and it offered substantial improvements over those. I rarely shot my D2X beyond ISO 400 and my preference was to keep ISO at the base setting of 100.
Well, despite the D90 being newer, I would think that the D90, being a slightly lower-end model, would be about the same as the D300 in performance. *shrugs*.
They're both based on the same Sony sensor, but the D90 being a year newer benefits from further development. FWIW, rumor has a D300 successor being announced sometime this year. It will undoubtedly leapfrog past the D90.