Okay, spent a bit of time on Newegg (pussy). This is a basic system that you should be able to strip back if you need to or at least give you a price comparison.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811112025 Lian Li case, fairly basic one but they are sexy as. $109.99 but there is $20 discount at the moment for whatever which brings it to $89
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153114 PSU, 700W. Good brand, specifics I am kind of meh on but didn't want to under power this, could possibly drop this back. Could easilly spend more on a PSU though.
$74.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131297 Asus Mobo socket 775 will fit a Core 2 Duo and is a brand and model I trust. On board audio and network cards and blah blah, standard fair. $189.99 but with $50 off for whatever reason $139.99 Apparently a mail in rebate but that shit is fucked up, what is wrong with Capitalism these days?
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115037 3GHz Core 2 Duo. Alright CPU, will do you. Could easilly spend more, could probably spend less. $167.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231283 2x2Gig kit of G.Skill Ripjaws RAM, actually a lower version of the RAM I am looking at the moment, apparently some really good cheap RAM at the moment. Speed matches the recommended MOBO and whatnot. Could spend more, could spend less if you went OCZ which used to be good but heard rumor of them flaking out a bit lately on RAM (think they may have shifted focus a bit more to SSD) $109.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150436 Video Card. ATI 4870. Don't know the brand at all, but I don't know the other brands as well. All these cards are going to be pretty hot, so if that is an issue you may want to invest in some nice fans and stuff. $154.99
Net price $737.94
Can shave cost off in places here and there, but there you go, a basic blueprint.
Edit - Assuming you don't need a new DVD Drive.
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106289 A whopping $30 so yeah whatever, can get one easy enough if you need it.
Edit 2 - Of course I didn't even bother really looking at the deals after I looked at a couple and saw amazing deals on really fucking shit parts, so yeah balls to that. Can do it if you want, but fuck me the offered bundle Mobo with the CPU was fucking shocking piece of shit.
Edit 3 - Oh yeah and if you are worried about your hard drive being fucked
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148411 is $80 this would all bring the price up over $800 at which point is way past what you wanted to spend (but all TOTALLY worth it I assure you). If you have a case that it will all fit in you -could- use it, but the Lian Li is sexy and a new case is always a good way to start a PC, you know for sure it is clean. If you had something melt previously you may want to try and make sure it stays so since dust clogging and killing fans is more likely to cause a critical parts failure than actual amazingly bad ventilation from my experience. If you leave your case on the floor right now I -highly- recommend that you get something to lift it up off the floor. If it sits on carpet the fan at the front of a case will just hoover dust up from the carpet right in front of the case like nobodies business. Even just a small length of wood larger than the base of the case will do (looks like shit though).