£700 Budget, best performance:cost ratio.

APPROXIMATE PURCHASE DATE: 0-2 weeks
BUDGET RANGE: £450-£550 + £100-£150 for a monitor (Ideally, I'd rather keep the total nearer £600)
SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: MMO gaming (multi client), web surfing...
PREFERRED WEBSITE(S) FOR PARTS: ebuyer.com, overclockers.co.uk
OVERCLOCKING: Maybe, never been a big fan.
MONITOR RESOLUTION: 1680x1050 or 1920x1080, having trouble picking
NOTES: I have terrible luck with component life span. Ideally I'd like a 2-3 year warranty on as many parts as possible. Reputable brands and those known to make good on a warranty.

Going with an AM3 build.

I'd just like to clarify I'm looking at getting the most "bang for my buck", I don't just want a cheap build. There is always a sweet spot in the market where you can spend a little more cash and get a lot more performance. Ideally that's what I'd like to be aiming for with all components.

Preliminary build so far is:

Asus M4A785TD-V EVO Socket AM3 onboard DVI VGA HDMI 8 channel audio ATX Motherboard £72.94
2x Crucial 4GB (2x2GB) DDR3 1333MHz/PC3-10600 Ballistix Memory Kit CL6 1.8V £109.99
AMD Phenom II X3 Tri Core 720 Black Edition 2.8GHz (Socket AM3) - Retail £89.99
Sapphire HD4850 1GB GDDR3 HDMI DVI VGA PCI-E Graphics Card £87.32
Western Digital WD7500AADS 750GB Hard Drive SATAII 32MB Cache - OEM Caviar Green £46.99
Samsung SH-S223F 22X DVD±RW/RAM/DL Serial ATA Black Bare Drive - OEM £16.25
Silver Mid Tower Gaming Case with Massive 14cm Front Fan - No PSU £17.99

That comes to around £441

PSU is missing, I'd love some advice here. Something that will power what is listed above, not much else will be added to the system with regards to power needed.

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I've gone for the tri core as a few suggested, as looking at the results of the article linked, I just don't see a 4th core being worth £50 more. I don't really mind the extra cost, I just want it to be money well spent. I am a prolific multi tasker and often run 2 games at the same time. So if this means I can play Crysis and an MMO of my choosing at the same time with a decent frame rate... that would be ideal. I wouldn't mind a little more feedback here. 810, 920,955 all look tempting but I don't want to be spending more than needed for the same results. Multi core support is still lacking so I'd rather go with a faster speed on the cores I get than simply more cores.
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I was quite certain about going with Saphire (3 year warranty and reputable brand), but PowerColor are offering a HD4870 for less than £15 more. Both cards have 1 gig ram available, how big of an improvement is the 4870 compared to the 4850.

I've never been too blown away by graphics, but frame rate is something I can appreciate! Fast and capable is what I'm looking for.
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Monitor...

I'm totally lost here, to a degree they eventually become the same. All 300/m2 10000:1 5ms etc. They tick the boxes for being bright, responsive with 16 million colours and a good contrast ratio. But if the vital statistics are all the same what is left to know if you're buying a good monitor?

Samsung SM2343NW 23" TFT Monitor 2048x1152 300cd/m2 20000:1 5ms VGA Black 3 Years Warranty

I'm quite drawn to that. It's VGA rather than DVI (is that a big deal? I've had mixed verdicts there).

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Thanks for reading.

Any advice and feedback to help save money if it is wasted, or get a little more performance for a few extra ££ would be great.

only a few tweaks to recommend. Since you have gaming on your usage list, you might want to choose a comparably priced WD caviar black drive over the caviar green. If you were doing more of an HTPC build, the green drive would have been right on. As for the case you selected, I try to stay away from generic cases, especially ones with very little info on them. Can't complain about the price though. Usually the really cheap generics are made out of thinner 0.5 mm SECC steel, kinda flimsy, but usually not an issue if it's not going to be moved around much, or worked on a lot. I usually like to see 0.8 mm SECC steel. Also, I bet the 13 cm fan is a bit louder than competitors. There are some customer reviews on that case at least so the possible flaws may not be so bad after all.

Although I myself am actually looking for advice on new build I did have my eye on this which may be a decent monitor.

BenQ G2220HD 22 Inch Wide LCD Monitor (16:9 Full-HD 1080P,5ms, 10,000:1,DVI) - Glossy Black for £131 doesn't seem too bad

Would Coolermaster Elite 330 Black Mid Tower Case - No PSU be a better option? I don't mind spending another £10 for a good case. My case will just sit under my desk, never be moved

I've switched out to a WD Black also. Appreciate that as I overlooked what the colours represented.

That is a much better case, though it might be lacking a 120mm fan to place in the front. The specifications seem to contradict itself on if it really has 2 120mm fans (1 front, 1 back), or if it just has one and the second is optional. If the case doesn't come with a front mounted 120mm fan, I'd recommend getting one. I would recommend getting a fan that's priced in the middle, not dirt cheap, but not really expensive. Antec fans usually sit right in the middle of the range.

As for HIS as a Gfx manufacturer, they are relatively new to the market, but I haven't heard any terrible horror stories from them. It's usually the smaller, less established companies that are willing to go the extra mile when it comes to customer support anyway.

cept the 4870 wich was a dressed up stock cooler