Dell do not make any Windows 7 drivers available for the OptiPlex GX620 even though this machine is still a perfectly capable workstation. Where I work they make up most of our inventory and they are all being upgraded to 3GB of RAM ready for the roll-out. The Pentium D CPU runs in x64 mode, and even the Pentium 4 HT 3.40GHz sold in early configs is 64bit capable.
Windows 7 includes a driver for the Intel 945G integrated video card, though Windows Update will suggest a newer one. There is an issue with this (see my post for details), so you should use version 126.96.36.1992 instead which you can find at Microsoft Update Catalog using a search string of Intel 82945G Express and sorting by version number. You’ll need to use Internet Explorer because it requires an ActiveX control to be installed. Download both of the two matches – one is x86, the other x64 but you can’t tell them apart until they are downloaded.
The SoundMAX Integrated Audio is detected and installed by Windows Update, but if you’re maintaining a WIM image or building an unattended install you’ll need to isolate the driver files. These are also available at Microsoft Update Catalog – search for SoundMAX Integrated Digital Audio, order by version and look for version 188.8.131.5210 for x64 and 184.108.40.20610 for x86.