Hi, I am currently building my own PC, so I've just done research into parts and components. Let me share my research with you.
Forget the other guy who told you to switch from AutoCAD to Corel -- you likely have your reasons for using AutoCAD. Maybe that's what your school or company uses. AutoCAD is fine and still the industry software standard for industry design works.
I recommend buying most of the parts and components from Newegg.com, which seems to have the best prices all around. Be sure to check out alternate sellers like Tiger Direct. Sometimes you can get better prices on specific items elsewhere.
I will list and add up all the prices at the end.
1. Intel Core 2 DUO 2.2 GHz -- The chip model number is E4500. It costs between $100 and $120.
Here's some tips: all currently available Intel Core 2 Duo and Quad chips use Socket 775. That means if you want to save money now, you can always buy a cheaper CPU now and then upgrade to a better CPU later -- as long as the both CPUs use the same socket and the motherboard accepts them.
By the way, Intel has just announced price cuts for most of their chips on April 20. So, you might want to wait a while for the retailers to adjust their prices accordingly. Most retailers are still using old price list. (I'm actually waiting for them to lower the prices.)
2. Memory 2 GB DDR2 Memory -- G. Skill 2GB (2 x 1GB) DDR2 800 offers the best price-to-performance ratio at about $50. Buy a pair (2 x 1GB) instead of one (1 x 2GB) to utilize dual memory channels.
Here's the ranking of memory manufacturers:
G.Skill
OCZ
Mushkin
GeIL
Corsair
Kingston
Crucial
They are all GREAT manufacturers that make GREAT memory. G.Skill and OCZ have the best bang for the buck because they offer some of the BEST memory at some of the LOWEST prices.
Mushkin and GeIL are very close behind those two.
Corsair, Kingston and Crucial have been around for a long time. They have solid reputation and most people know them. (Unlike G.Skill, which is not too well known outside the enthusiast circle even though G.Skill makes the best memory for incredibly afforable prices.)
If you do decide to go for G.Skill, do not buy it on eBay. There is an eBay seller named "Wong" who is ripping people off by selling overpriced G.Skill memory.
3. Harddisk 160GB SATA Hard Disk: Either Samsung or Seagate is a good choice. Seagate seems to have a better reputation than Samsung. You can get either one for about $50.
Forget the other user who just tell you to spend, spend, spend and get the biggest, faster drive now. You can always add a second hard drive later. In fact, there are certain benefits to having TWO instead of just one hard drive (i.e, you can use the first partition in the second drive to cache memory for overall better performance, you can minimize the risk of loss data, etc., etc.)
4. Mobo Intel D945GCNL: About $70.
I'm not familiar with this motherboard, but I hope you can upgrade from duo core to quad core with it.
I recommend spending as much as possible on the motherboard. Why? You can always upgrade the CPU later. You can always drop in more memory later. You can always add a second hard drive later. When you add new parts and swap components later, you will be able to keep your original system, setting, programs and data, and you don't have to reinstall Windows. It is still basically the same system.
However, if you replace the motherboard, that means you will be getting a whole new system. You will have to reinstall a new Windows -- and since it's a new system, Microsoft won't let you use the same key code. Once you change the motherboard, everything is gone. You start all over again with a new system.
5. Samsung SH-S203N (with LightScribe) seems to be getting the best user reviews -- I've checked multiple forums and discussion boards. It's better than Sony's or LG's. And Samsung is about the same price (one of the cheapest) at $30.
The other good brand is Pioneer. LiteOn burners are comparable to top-brand burners, and they're cheap.
Avoid Blu-ray drives at the moment because they are still too expensive and too slow. Do you know any PC software or game that requires a Blu-ray drive now or in the future? NO. Ignore all those Blu-ray cultists and fanatics who will try to convert you at your expense. I'll get a Blu-ray drive/burner when it's fast and under $50. Right now, it's just not worth it.
6. Internal Modem Conexant / Creative: I'm not sure why you are still using modem, but you can get it for about $10. Unfortunately, I can't find the Creative Conexant modem -- modems are becoming obsolete and harder to find nowadays.
Let's tally them up using prices at Newegg.com:
Intel Core 2 Duo E4500: $119.99
G. Skill 2GB (2 x 1GB) DDR2 800: $46.99
Seagate Barracuda 7200.10 160GB SATA: $50.99
Samsung 20X DVD±R DVD Burner with LightScribe SH-S203N: $30.99
Rosewill RC-403 Conexant 56Kbps PCI Bus Modem: $11.99
Total: $260.95
That does NOT include shipping. Newegg will try to ship the entire order together to save on shipping cost.
Since you are running AutoCAD, you will likely need a video card (unless you already have one and plan to bring it over to the new system.) Here's a good reference that's updated every month: http://www.tomshardware.com/reviews/gaming-graphics-cards,1805.html