The good news is that motherboards still exist that support your current components. The bad news is that there are very limited options when it comes to these boards and none of them would be a perfect match.
Here are the options I was able to find:
MSI 848P Neo2-V LGA 775 Intel 848P ATX Intel Motherboard This board supports your CPU and RAM but is full-ATX so you will need to replace your case. This is also a recertified board.
Intel BLKD975XBXLKR LGA 775 Intel 975X ATX Intel Motherboard This is the only brand new, non-refurbished board I was able to find that supports your cpu but it would require a new case and new RAM (DDR2).
Honestly unless your budget is extremely tight your best bet is a new system given how outdated those components are.
If you reuse your current HDD and optical drive (if it is SATA) and don't need a dedicated video card you could build a good system for around $280
CPU:
AMD Phenom II X2 555 Black Edition =$88
Motherboard:
MSI 760GM-P35 AM3 AMD 760G = $60
RAM:
Crucial Ballistix sport 8GB (2 x 4GB) 240-Pin DDR3 =$48 (newegg also has corsair for the same price but it is currently out of stock. If it is in stock I would recommend the corsair but crucial is a good brand as well)
Case:
COOLER MASTER Elite 371 = $40
Powersupply:
Antec EarthWatts Green EA-430D Green 430W = $45 (you can also get the corsair builder series 430w for this price but I would recommend the Delta made Antec over the Channel Well made Corsair.)
Extras:
Optical Drive:
LG CD/DVD Burner 22X =$18 (if your current ODD is not SATA)
Hard Drive:
Western Digital Caviar Black WD7502AAEX 750GB =$59 (if you choose not to use your current HDD)
Video Card:
SAPPHIRE 100338L Radeon HD 6770 =$110 (if you have been using AutoCAD with your old system without a problem the integrated video on the motherboard listed above should be sufficient although most people who use AutoCAD or do allot of video editing like to have a good video card.)
Also you may need a windows license: windows 7 home premium OEM goes for $100 and professional is $140