Hi! back :P
Since the games you're approaching are not as graphic draining as it is, I believe your computer will only need to focus on the requirements of your AUTOCAD programme. lets look at the requirements:
Microsoft® Windows® 7 (compare Windows Vista versions); or Microsoft® Windows® XP Professional or Home edition (SP3 or later)
For Windows Vista or Windows 7: Intel® Pentium® 4 or AMD Athlon® dual-core processor, 3.0 GHz or higher with SSE2 technology; for Windows XP: Intel Pentium 4 or AMD Athlon dual-core processor, 1.6 GHz or higher with SSE2 technology
2 GB RAM
2 GB free disk space for installation
1,024 x 768 display resolution with true color
Microsoft® Internet Explorer® 7.0 or later
Install from download or DVD
So maybe in my opinion, 8gb ram may be took much but even I tend to feel reassure computer lags has nothing to do with my rams if this a pile of them. :P But going for rams you'd have to buy ddr3 since they're now more vastly used then ddr2, i'd choose brands such as corsair and kingston, but I'd refuse to get the HyperX from kingston. they don't seem to be nice and very fragile... also you need to check your motherboard compatibility with the rams :P.
Now on to the cpu, since various rivalry came between the Phenom II X6 and the intel Core i7 2600K many would prefer the i7 for that extra POWER however if you choose this path you'd be paying around 80 pounds for for unnecessary power :P in my point of view anyways. I too like the Phenom II X6 since 6 cores really does allows you to multi-task at a new level of brilliance.
The graphic card we'll have to check with everyone's opinion because after this reply, there may be a huge war or a silent one in the choice between Nvidia or ATI, personally ATI seems like a more vaild choice for me xD In long term you seem to get less problems with it after all, anyhow recomending a few:
-HIS HD 6790 IceQ X Turbo 1GB GDDR5
-Sapphire HD 6770 1GB GDDR5
These two are a few of my favourites since the first choice, this card runs really cool and it works well if you're planning to play double graphic cards on a AMD board called 'crossfire for ATI' (although crossfiring is slightly unneeded :P ! note:
Looking at graphic cards people would normally aim for one topic, the clock speed, this how fast the graphic card can run and fire the frames in. You could also look at the around of memory it has too, the memory is how much frames the graphic card can hold before firing them on your screen. P.s if my form of explanation is wrong or confusing, i'd be happy for anyone to correct me.
Since I have less experience in nvidia I'd prefer someone else to give you more advice on that
Okey, to the motherboard. On the view of you taking the AMD route i'd say.
-Asus M4A88T-V EVO
Asus boards have a really stable chip set so it won't die as easily, plus they have something call a turbo key (This you should check out yourself because its REALLY cool
Your CPU cooling fan I'd really recommend: Zalman CNPS9500A-LED Aero Flower its amazingly nice cooling (also you wouldn't need to use a hug fan since you're using amd which doesn't produce as much heat as intel) and it also looks pretty good too
Plus its really silent!
Now the PSU (power supply!) I found the Coolermaster GX 650W PSU one of the best with a solid strong build (if you're looking at psu the heavier the better if you can lift it up in front of you) Its also small compared to the other psu's which makes it easier to fit in your case! alot of reviews on this psu so check it out!
This should only leave us with the case: which in my opinion I cannot recomend for you, it comes in various styles and lighting which cover a vast majority of people who like them :P so its personally preference in choosing case as well as how well it is built or how silent the fans on it is.
All that is really left to do is the little bits such as buying windows, or thermal compound, ect.
And OF COURSE! for you to choose the ingredients inside your computer case :P Good luck! if you have any other questions be sure to ask, we don't bite
(I hope o.o)