Dell Optiplex 745 - Max CPU RAM etc

Depends on which model you have.  The USFF is the least upgradeable due to the smallest power supply and only 2 Ram Slots instead of 4.

Bios Should be updated BEFORE trying a new CPU.

Intel® Core™2 Duo Processor E6700 (4M Cache, 2.66 GHz, 1066 MHz FSB)
Intel® Core™2 Duo Processor E6600 (4M Cache, 2.40 GHz, 1066 MHz FSB)
Intel® Core™2 Duo Processor E6420 (4M Cache, 2.13 GHz, 1066 MHz FSB)
Intel® Core™2 Duo Processor E6400 (2M Cache, 2.13 GHz, 1066 MHz FSB)
Intel® Core™2 Duo Processor E6320 (4M Cache, 1.86 GHz, 1066 MHz FSB)
Intel® Core™2 Duo Processor E6300 (2M Cache, 1.86 GHz, 1066 MHz FSB)

Memory Type: DDR2 PC2-5300, DDR2 PC2-8500, DDR2 PC2-6400, DDR2 (non-ECC)
Maximum Memory: 8GB
Slots: 4
Each memory slot can hold DDR2 PC2-5300, DDR2 PC2-8500, DDR2 PC2-6400 with a maximum of 2GB per slot.*

Recommended Upgrade

System Type: OptiPlex 745 Ship Date: 3/2/2007 Country: United States

Quantity Parts # Part Description 1 JH313 Assembly,Cage,Riser,Matrix neo,desktop,Pc Interface,DUAL 1 UH261 KIT,DOC,TSH,SVC/SPRT,CLNTGTS,T 1 KN408 Hard Drive,80G,S2,7.2K,8M Unleaded,Seagate-Tonka 0 149DF INFO,PREP MTL,DEV,PWS,INCRS,#2 1 UH650 Assembly,Floppy Drive,1.44M Sony,05,Black 1 5120P Cord,Power,125V,6Feet,SJT, Unshielded 1 YK181 Guide,Product,Information Client,DAO/BCC 1 CT739 TSH,RBTE,VISTA 1 UT879 Kit,Software,Overpack,WXPPSP2 Compact Diskette W/documentation,English 0 46022 INFO,ENABLE GRN PC,MS-DO 0 92026 INFO,HMEDIA,READ ME,E-STAR EN 1 2J973 PREP MTL,SRV,DELETE DOC,NONINV 1 YH933 MSE,USB,2BTN,ULD,NTRY,PMX,BLK 0 83535 INFO,PART,ATAPI,DEVICE 1 XH527 Assembly,Compact Disk Read Write/digital Video Disk DriveCombo,48,Sony,Serial Ata,Black 1 DJ331 KYBD,104,USB,US,NTRY,LTON,BLK 2 PN424 Dual In-line Memory Module,1G 667M,128X64,8,240,2RX8 1 D7931 Assembly,Heatsink,Shroud Mainstream,MND 1 JT808 INSTR,DEVIATE-TO-MND-L5.5,745 1 PN099 Processor,915,2.8,2X2MB Pentium D Presler,C1 1 KX123 ASSY,CHAS,MND,OPTI,745,L6,V2

What can be done is listed above.

It was taken from the spec sheet.

Dell™ OptiPlex™ 745 Tech Specs

If you have a 32bit version of Windows (almost all Windows XP were 32bit, Vista and 7 could be either 32 bit or 64 bit versions) then Windows will not be able to address any memory over 4gb.  In fact from the 4gb of addressable space other memory space such as the amount of memory on the video car is subtracted from this maximum addressable space.  Take this into consideration before spending money on memory that you version of Windows may not be able to take advantage of.