Ok jesus christ, sorry murlo just had to say that, you say you’re an engineer, you know how to build comps, but all these questions jeez yourself......
.....but to make my splattering of rudeness more confusing I totally agree with your situation, I am very knowledgeable when it comes to comps. BUT I love to hear other suggestions and opinions on how and what I should build, so I don't think you Q's and A's are dumb. (This response is probably belated, but I just read through this whole thread and it’s a little ridiculous).
Ok to your last few questions, you can (regardless of what ANYONE tells you) upgrade any dell machine, with ANY parts you want. BUT and this is a BIG BUT, once you do upgrade you may NOT be able to use the recovery disk that comes with the PC, as it is usually programmed to work with the specific parts that are designed into that model, saying that, there is no reason why you can't just buy an O/S by itself and install that.
But after saying all that..... let me give you some REAL advice, buy all the parts yourself and build the PC yourself, it is really not very hard AND, you can buy far far better stuff. I see the 8400 is about $1000, for that you could build a real nice PC. I can build a faster PC, than alienware can for under $1000, Alienware usually charges at least $3000. If that’s any indication of the markup PC co's charge.
If you need any help, I’ll research all the parts and show you exactly what and where you can buy what you want on a certain budget, and if need be walk you through building one, if you're a little unsure. Also it’s a lot of fun building it yourself; it is also a lot easier to fix something you built also.
Let me know. If you were really a retard, then you should buy a commercial pc, but otherwise and it soundsto me like you are more than capable to build it yourself.