Ok, a 300watt supply is pretty small, but, there are a few cards that will work fine with it. These cards will provide Excellent 2D performance for desktop work, websurfing, and videos. Any will be far better at 3D support while they will offer some support for older games at lower resolutions, (again, Far more than you have now....).
http://www.realhardtechx.com/index_archivos/Page362.htm
Choices: 4550, 4350, 3450, GT220, GT210, 9600GT, 9400GT, 8400GS,
Found this additional info on your systems motherboard, included it here JUST for reference:
Product Description
This is a Genuine Dell Optiplex 745 Desktop DT Motherboard Intel Q965 (ICH8) Express Chipset – Supported Processors: Intel Core 2 Duo 1066MHz FSB Socket T with Dual Core technology XD, EM64T, 2MB and up to 4MB L2 cache, EIST and VT (E6000 series); Intel Pentium D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x2MB L2 cache, EIST, and VT on 950/960 (900 series); Intell Pentium D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x1MB L2 cache (800 s… More >>
Supported Memory: Four DIMM Slots; Dual channel shared DDR2 SDRAM system memory. Unbuffered, non-ECC DIMMs only; 533MHz (4-4-4 latency), 667MHz (5-5-5 latency), 800MHz (6-6-6 latency); 512-MB, 1-GB, or 2-GB (800MHz not available in 2GB DIMM)
Peripheral Interfaces: VGA; Six USB 2.0; Serial Port; Parallel Port; Ethernet Network; Stereo Line-In; Speakers Line-Out;
Expansion: 1 PCI-E X16 slot, supports x16, x8, x4, and x1 modes/cards; 2 PCI 32Bit / 33MHz slots; 1 FDD connecto – Audio: Integrated – Video: Integrated
Dell Part Numbers: HP962, KW628, PT395, RF705, MM599, WW034,YJ137, NW444, NX183, KW628
Supported Processors: Intel Core 2 Duo 1066MHz FSB Socket T with Dual Core technology XD, EM64T, 2MB and up to 4MB L2 cache, EIST and VT (E6000 series); Intel Pentium D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x2MB L2 cache, EIST, and VT on 950/960 (900 series); Intell Pentium D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x1MB L2 cache (800 series); Intel Pentium D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x1MB L2 cache (800 series); Intel Celeron D 533 MHz FSB Socket T with XD and EM64T (300 series)