Am pretty sure(99%, but not positive) the processor in your 8300 is a socket 478, the mobo you're looking at is LGA 775 as all mobos for new Intel's will be. so, you cannot transfer your processor to the new rig.
The memory in your 8300 is DDR which is 184 pin.
Most all of the new mobos will be DDR2 which is 240 pin.
So, once again you will not move your existing memory to the new mobo.
The only items you are going to be able to scavenge from the Dell will be any 2 of the following 3 items. The CD, DVD or the hard drive. All 3 of these in your 8300 are going to be IDE devices. The Asus board will only handle 2 IDE devices - designed for your CD/DVD drives, but you could put a hard drive on it. It has 6 SATA connectors for hard drives.
I hope you are beginning to see your dilemma. The hardware you have listed above at $700, still is no good to you without also buying a processor, a hard drive and an operating system. And then scavenge the CD/DVD drives from your Dell. And you have a pair of what, for sure in 6 months, will be mediocre video cards (the hardware enthusiast would already consider them to be mediocre).
I know you have the fever (the hardware bug bit you nasty, huh), but once again, if advice is what you're looking for I would say spend about $75 for another Gig of RAM, am assuming the 8300 owners manual online is correct in that you have 4 slots available.
Here is some corsair that would fit the bill. Then check
here for video cards. Stay in the $100-$150 max price range maybe with a nVidia 7600 or an ATI x1600 or 1650. This'll take care of your immediate fever.
Set aside the rest of that $700 and start saving and a year from now when those DX10 games hit, you could build yourself a killer up to date gaming rig.