What's the fastest GPU I can get to fit in a small PC?

I don't think you can beat that. But...
What PSU do you have? Where'd you get 100W from? That card uses a 6-pin connector, which is technically a molex, but pretty much never referred to as such. I just want to make sure you know it's not a 4-pin. I doubt it's worth 100 pounds to upgrade though--the performance difference is only 15% or so.

You'd do better saving the cash and overclocking the 5670 and turning down the fan. That of course would heat the card up--but might get you what you want. You'd want to monitor temps. The Catalyst Control Center should let you do that stuff.

HD 5770 - 100w at peak 3D and 78w at typical 3D.
http://forums.atomicmpc.com.au/index.php?showtopic=264

$150
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Not sure about length or where available in UK

check manu for length
http://www.scan.co.uk/products/1gb-xfx-hd-5770-pci-e-21...

Wait! 810mm? You should check that because that's 32 inches! The biggest cards are 12 inches (300mm) or so.

And if you're open to 108W, the 5770 is a better performing upgrade. It can totally play any game and medium settings or so.

I've looked at the 5770 and it is just slightly too long in most configurations I've seen. Some of the boards from lesser known manufacturers look smaller in the product pictures but they don't list dimensions on their site (helpful).

The performance difference between the 5750 and the 5770 is minimal but potentially worth having... I shall keep hunting for one that fits!

Sorry I meant 180mm.

My PSU is a Dell OEM part with no 6-pin connectors, but I can easily use adapters to get the required 6-pin output from it. It's a 280W PSU, and the system is designed for low power usage. It uses a 65W CPU, a high-efficiency motherboard and a single SSD. When it's on full stress-test mode with the onboard GPU enabled it sucks about 110W from the wall, which leaves me with approx 130-150W headroom at full load. A 100W peak card would fit nicely in this system without taxing the PSU at full load.

Overclocking a GPU is pretty worthless in my view. All you get is a hot unstable system and 10-15% more performance at best. When you overclock you raise the frequencies, you don't add any more cores or stream processors. (5670 = 400 cores, 5750 = 720 cores, 5770 = 800 cores)

I'm not really worried about the cost of the card as even a top-end 5770 is only about £120. If anything I'd rather get a more powerful card with more cores and underclock it to save on noise, heat and power consumption.

Overclocking a GPU is VERY useful--but not good for your situation. It's often easy to get 10-15% extra out of them with complete stability--even with no voltage increase. I've gotten 35% extra out of some cards.

Yeah...180mm is only gonna fit some special HD 5770's. The Powercolor 5770 is 182mm (http://www.powercolor.com/us/products_features.asp?id=2...), but it's got a rear-facing connector.

Check http://extreme.outervision.com/psucalculatorlite.jsp to see if your PSU is good enough. 250W is REALLY low to run a decent graphics card off of--so be careful.

Sorry to say... but you are stuck with the HD 5670.

I do not recommend anything more powerful than the HD 5670 because of your weak power supply. If you want to risk overloading your PSU which can then create an electrical surge which can potentially destroy at least one component, then go ahead.

What's important is how much power the 12v rail provides since most components draws power from that rail like the CPU, Hard Drives, SSD, Optical Drives, Fans, and the Video Card.

Well it's 280W not 250W...

I have very little hardware in my system, 1 fan, no optical drives, no hard drives etc. Here are the manufacturer rated peak wattages:

65W Core Duo CPU
20W Intel Q35 motherboard & chipset
10W OCZ Vertex 2E SSD
5W Dell system fan.

I make that 100W peak load, which would be 122W from the wall if my PSU was running at it's rated 82% efficiency.

I've measured the peak power draw at the plug socket using an energy meter. I saw ~115W with no GPU and ~180W with the 5670 installed, so my calculations seem to be about correct, and in fact slightly conservative.

The 5770 is only rated 40W higher than the 5670, which would put me at ~220W at the wall, well within specification.

Also it's worth bearing in mind that these are *peak* wattage calculations, so when the CPU, SSD, Graphics card and system fan are all totally maxed out. This does not happen at all often in games, even during a heavy COD Blackops session.

As for overclocking a GPU being "very" useful, I remain unconvinced. A 15% increase in clock frequencies equals (optimistically) a 7% increase in game performance, perhaps 2 or 3 FPS with vsync turned off. Would I even notice that? Probably not. Would I notice the noise of the fan on the graphics card spinning faster? Definitely.

i think this is the most powerful single slot graphics card but you would need a better PSU to handle this http://www.scan.co.uk/products/1gb-kfa2-gtx-460-razor-3...

And a much bigger case, that one looks like it's about twice as long as will fit in mine!

If I had a case big enough to fit that monster, I would probably also have enough space for a nice quiet dual slot cooler...