Could someone recommend a half-decent graphics card (for light gaming) which costs under £50 and fit's the Antec NSK1380. I've come up with the following so far:

1) Sapphire Radeon HD 3650 512MB DDR2 - £43
2) Sapphire Radeon HD 3650 512MB DDR3 - £50
3) Sapphire Radeon HD2600 XT 256MB DDR2 - £50

Any of those sound like a decent choice?

The 3650 looks ideal for you. You can also look at one of the cheaper 4650's, they are slightly more expensive but offer much better performance. Not many of them around right now though, give it a few weeks and im sure we'll see more of them.
Here's one from novatech. That fan doesnt look too quiet, but since the card runs so cool you can probably turn that fan down to just above stalling and it would still keep the card cool.

If I had to pick from those three, I'd choose the 3650 with DDR3. However, that model has a fan, and this GPU can easiliy be ran passively. Judging by these two builds, having a large heatsink on the graphics card is possible. So I found a GDDR2 equipped 3650 and a Arctic Cooling S2 for a total of £50.75 including VAT. This will give low end gaming performance with no additional noise.

For 65 quid including VAT you could get a HIS 4670, it should be much faster that a 3650:
http://www.overclockers.co.uk/productli ... subid=1274
And it exhausts most of the generated heat through the back vent.

Thanks for the info guys, is it worth spending the extra few quid to get DDR3 memory on the gfx card or will it not make much difference?

BTW- will adding the S2 cooler void the warranty of the graphics card?

What do people make of the ASUS EAH3650 SILENT MAGIC 512MB graphics card. It says 0db but i'm not sure if it would fit in my case/motherboard (Gigabyte GA-G31M-S2L or Asus P5KPL-AM)

I've seen many, many people ask this question on nearly every computer enthusiast forum. My personal take on this is that it really doesn't matter. If you need warranty work you could just put the stock cooler back on the card. As long as you don't physically damage your GPU, there's no real way to tell whether you've been using an aftermarket heatsink.

I'm just about to build a system in an NSK1380, but I need to go with an nVidia-based card. I was looking at a fanless 7300GT from Asus. Will that work in this enclosure, or would there be heat problems? I asked Antec and they told me to ask the video card manufacturer. There doesn't seem to be any way to contact Asus without buying something first.

I was also thinking of a silent Asus 8400GS, or an EVGA 8600GT with fan. I'm limited in my choices because I'm building a Hackintosh and I need hardware that works for that.

I am currently using a Sapphire HD3850 ultimate in one of my two web surfing boxes the other one had one too but it has died . So I too will be looking for a low to mid range passive cooled card if I cannot find another Hd3850 ultimate either new or S/H (eBay most likely).
I have been looking at the Asus 3650 and have been hoping someone will bring out a passive 4670 or maybe even a 4830 (wishful thinking ) . The machine is currently running 780g onboard graphics .

This has got to be on the cards IMO, it's written about a lot on forums and is totally practical considering this cards power draw. If they build it, we will come - only a matter of time I reckon guys.