Well, as I started L4D today, after downloading for 5 hours and full of appetites for it, I had to see that my supicious' were totally true. My computer is not made for this game, so after I was thinking for some time to buy a new one, I will finally do so and I'm willing to spend around 1000€ maybe a bit more.
Due to my lack of knowledge (yea I'm pretty much a nub ) concerning new hardware and how to compose a working system I ask u for concrete suggestions.
I need everything but a mouse, a keyboard, a headset , (not sure about) a soundcart.
I think a new monitor would be pretty good, too.

This is what I'm having atm.
Please help me.

Alright, rough suggestions:

one real fast Intel Dual Core 2 Duo (right now I don't think it's just wise to spend lots of more cash on quads)
one bunch of 4GB of Corsair ram
one Gigabyte motherboard
one Radeon HD 4870 (this is price for performance. If you want higher performance (but also higher price) go for geforce)
one big hard drive (or if you don't consume lots of space, go for speed - I have a HD Velociraptor 10,000 rpm drive)
some kinda disc player which plays round things that can also be rectangular
floppy drive (not really, but I suggest you save one and a cable for urgent times)

And another thing I would suggest is some kinda case with good airflow. And possibly a third party cooler (not saying the one that comes with the processor is bad, but better is better)


or you could just go with less ram if you intend to use 32bit version of windows. Remember, the GPU memory stacks to the RAM. I think.

Unlike Murk i recommend a quad core processor, i picked my 2.4ghz Q6600 for only 120 pounds!

I'm running...

Intel quad core Q6600 2.4ghz
Geforce 8800 GTX XXX 768mb
Corsair 4gb of ram (effectively 3gb thnx to 32bit system)
MSI P7N Diamond nForce 780i

Not sure what else to recommend as i'm not sure what products are good., went with an MSI motherboard which is supposed to be good, although i get random BSOD due to my NVGTS.sys (or similar) which after hunting down is SATA.. hmm... doesn't like my Sata dvd drive.

I went with the Antec P180 for my tower, and it's stupid. It's a) monolithic and b) not enough actual space in the bottom, i had to take out the lower fan due to there not being enough room for my cabling..
Not a recommended case by me unless you really need the space and are pretty good at system building.

Sorry i can't really be more help since i just researched what i wanted, i do know that my pc came to 650pounds/824euros (Cost 210 pounds for the monitor, i dnt include that though but since you need one) and it runs Half life 2 benchmarks at over 170FPS

Plays new games coming out at full spec smoothly at 1680 x 1050, i haven't tested DX10 stuff though, i went back to xp after vista 64bit annoyed me greatly

For a monitor i went with the Samsung Syncmaster 2232bw (22") "pebble". Beautiful piece of kit, no dead pixels (same for my samsung 40" tv) and looks great at 1680 x 1050.
My MSI mobo came with a soundblaster card with it, but it';s not technically the "full" card so it's probably better to pick up a dedicated sound card.

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Computer is fixed. That is all.

Better wait until the new processors are coming or just spend not that much on a dual core now to later buy the better new processor.
The video card is just awesome, even if it's single gpu. Best in the range would be 4870x2, but the 4870 still pwns ass heavily.
Ram, well, any of the above would be good enough, just be sure to check out if you have quad or dual channel mem and if the 4 dimms or 2 x 2 gb work with 4 4 4 12 latencies.
Motherboards: This is quite personal, to be honest, don't spend anything less then 100 euros on this. Since I'm working with high end range boards, I never wanna go back to those crappy 50 euro boards.
The only thing I really put some more notion to are the HDs. The HDs are almost always the bottleneck of system speeds and such. Graphic cards and cpu make up for the fps in games and such, but load speeds and almost any speed in windows is made up by how fast your HD is (at least with the cpus we're talking about).
You could go for what saint bought lately, the velociraptor and for data just put a samsung spinpoint F1 500 gb next to it. Or you could make it a bit cheaper and just put 2 samsung spinpoint F1s in a raid0 array, that'll do the trick as well.

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M.A.S.K. , is the mighty power that can save the day