I'll take a stab at all these posts and questions with some advice. Not end all be all, but a few hints and tips when it comes to purchasing hardare.
Blitz:
Looking at the rig you mentioned I had a few suggestions. First off, why the different HDs? If your going to put them in a Raid 0 setup, you should really just get 2 of the same. The Seagate gate one you mentioned is quailty by itself too, that's alot of space. Remember if you place a set of SATA drives in a RAID0, you get twice the performance because it splits the data across both drives. This however means if one drive goes bad, so does your other.
Also your videocard I think will leave you wanting more. You really should consider at least a Nvidia Vanilla 6800 if your looking at mainstream cards. It has 12 pipes and the core is only a little off the GT models. In Doom3 benches it even beat the X800Pro. It smokes the 9800 pro in all benches as well. If you got that 9800Pro you'd be GPU limited in a lot of games, don't handicap your rig like that if your going to spend the money. Perhaps just ditch a gig of that ram instead.
Processor wise, if your going to go Intel, and your intent on playing Doom3 alot you may want to actually consider a Prescott core instead. The extra L2 cache size does provide a performance boost. I believe I read it on Tomshardware.com. Otherwise, in just about every other title I'd suspect the Northwood you chose would be fine. As mentioned as it is an OEM chip, you'll need to consider a cooler. I personally would just get a retail box that comes with a cooler if your not going to overclock. Otherwise, look at a Gigabyte 3D Cooler Pro. It lights up cool, and has great performance from every review I've read.
Jago:
Do not purchase any of those cards unless you want to play Doom3 at it's lowest and ugliest settings. They all have slow clocks, and low memory bandwidth because of the 128bit memory controllers (or something like that). What kind of system are you running on now, if it's a older rig you should maybe look at a Nvidia 5700 , or 9600. The 9600s tho I believe are mostly just 128 meg cards. Performance wise you should read the articles on Firingsquad.com about mainstream cards and their performance in game. Save yourself some more money before purchasing a videocard.
You can look at the performance benches at
http://www.firingsquad.com/hardware/doom3_pt3/ . Blitz you should read that too.
As mentioned before there are a lot of guides out on the net for Doom3 performance on various topics. Below is just a quick list of links that you can read up more. Later I'll compile a list of HL2/Source benches that were done recently on the CS:Source port.
http://www.firingsquad.com/hardware/doom3_perf/
http://www.firingsquad.com/hardware/doom3_perf2/
http://www.firingsquad.com/hardware/doom3_pt3/
http://www20.graphics.tomshardware.com/graphic/20040809/index.html
http://anandtech.com/video/showdoc.aspx?i=2146
http://www2.hardocp.com/article.html?art=NjQ0
By no means comprehensive, but all those together should really inform you on upgrading your videocard if your mostly interested in Doom3 performance.
Till next time