1) Not really. First because the X6 wouldn't be
that big an improvement in processing power [
compare]. Second because you'd still have just SATA2. Third because you'd still be sitting on 4GB of relatively slow DDR2 RAM which will bottleneck that CPU improvement a bit.
2) If my budget were exactly $300 for CPU+board+RAM, I'd shop like hell to find combo deal or sales that would net the following:
2x4GB (8GB) DDR3 1600 RAM
Intel i5-3570, 3550, 3450 CPU (in that order)
A Z77 (preferred) or H77 board
It's pretty tight but any kind of deal should get that happening at $300. Any of those chips at stock speeds are monsters.
2a) Make your budget $400, and you could go with an i5-3750k, 8GB DDR3 RAM, a $30 aftermarket cooling fan, and a modest but very serviceable Z77 motherboard. That gives you just about as fast as fast gets and comes with all of the goodies like SATA3, USB 3.0, and the option to use Intel's best integrated graphics package which shouldn't be too far off of your current card. And you can overclock the chip a fair ways which isn't an option without a "k" chip.
2c) There are other options for less, such as an AMD FX-4100 quad or better yet 6100 6-core. Not quite in the same performance class as the i5 but very good chips and priced well, as are the baords. Your CPU/RAM/board would come out to about $250 and be more than suitable for AutoCad and usually have your SATA 3. Add a little for a good cooler for sure.
3) Your PSU brand is about as obscure as they come, and from what I can tell is only available in Europe. The few reviews on it I could find were mostly good and it doesn't figure to be that old, so there's that. I'd be nervous trusting nice components to it but that's how I am about power supplies.
4) It depends on how you feel about the "other" parts. Do you trust the power supply? Do you have a retail or OEM copy of Win7 of your own, or is it a Dell disc you won't be able to use? Does the case work for you? Do you have a usable SATA-based DVD drive or does it connect with old-school IDE? I'd say if the rest of it is usable then upgrade, if not buy/build something else.