The Dell Optiplex 745 is a fairly robust little machine, considering its age. It is intended for commercial environments, therefore not a hobbyist's machine. It does not come with overclocking abilities in the BIOS. Like Rogue Leader said, you will have to download third party overclocking software. The Ultra Small Form Factor (USFF), the Small Form Factor (SFF) and the Desktop sizes all have minimal air flow and require a low profile mount bracket for add on cards. The Desktop version could have a PCI riser allowing you to mount a full height card horizontally. The Minitower is your best bet for overclocking as it has the better airflow design for more peripherals.
There are also five different CPU families that were shipped with the 745. Stock specs are as follows:
Celeron D 300 series, Single Core, 533 MHz FSB, ranging from 2.1GHz to 3.60GHz
Pentium 4 500 series, Single Core Hyperthreaded, 800MHz FSB, ranging from 2.80GHz to 3.80GHz
Pentium D 800 series, Technically a Dual Core (Intel's prototype before a Core 2), 800MHz FSB, ranging from 2.66GHz to 3.20GHz
Pentium D 900 series, Technically a Dual Core (Intel's prototype before a Core 2), 800MHz FSB, ranging from 2.80GHz to 3.60GHz
And my preferred for this model,
Core 2 Duo E6000 aseries, Dual Core, 1066MHz FSB, ranging from 1.86GHz to 3.0GHz
I have built many of the Optiplex 745s at my shop and the Core 2 Duo model, particularly the 3.0GHz Core 2 Duo is the cream of the crop. Maximum memory supported is 8GB of PC2-6400U, 800MHz (2GB x 4), with the exception of the USFF, which can only go up 4GB since it has only two DIMMs.
This computer will run 64-bit Operating Systems and I have installed Windows 7 on hundreds of them. I have seen the Core 2 Duo E6850 (3.0GHz) for as low as $8 before.
As far as the overclocking goes, the best advice that I can give you is to increment it slowly. Baby steps is key when experimenting with overclocking.