Would this CPU be a sensible buy?

That pentium D uses the old netburst achitecture. I wouldn't recommend it. 45 pounds is about $70 american and in america for that price you could get a much more powerful pentium dual core based on the core 2 architecture:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I'm sorry I don't have any links to UK sites.

Here is the dell service manual. Will tell you everything (hopefully):
http://support.dell.com/support/edocs/systems/dim4700/e...

Some older boards do not support newer 775 cpus. Be sure to check CPU support with dell.

If your motherboard supports 65nm CPUs, see if it supports an older Core2Duo type CPU such as an E6300, E6400, E6500, etc

Yeah, I know my system is a bit sketchy but I'll only be at home about another year and it'll be able to play most games at above minimum settings, so that's all I'm going for. Also I'm a student so I'm flat broke most of the time. So a new system is out of the question... But cheers for the info guys.

i can asure you that THIS is much faster than the pentium D 3ghz but i cannot tell for sure that your motherboard supports it.

I'm assuming your motherboard is a 915G chipset. I know those don't support Core 2 or Pentium Dual Core (Core 2's cut down little brother). Most 9xx chipsets don't (and ones that were being made while P4s were popular definitely don't). If they did, I'd be running Conroe right now instead of a D 930.

Two other questions: Can you power it and can you cool it? I'm not sure what the TDP on the P4 I assume you're running is, but that D 930 has a 95 Watt TDP and OEM computers tend to ship with smallest PSU that will reliably work.

A PD is a huge upgrade from a P4, but I don't think your motherboard knows what to do with one.

OK... Power is fine, got a decent PSU. And as for cooling That'd probably be alright, got a fairly good heat sink with it. So are you saying if I plugged this into my MOBO then it definitely wouldn't work?

Lol, based in england here so no college. But in upper sixth doing A levels. 18 atm. But yeah... what'll happen if I plug a dual core into my mobo that doesn't support dual core processors... Will my PC just cry and BSOD?

Could you tell us what exactly you are running now? If you don't know, download CPU-Z and run it. It'll tell you the CPU, motherboard (chipset is what we're interested in), as well as memory type and graphics card.

Then we check Intel's ARK to see if they're compatible. It's HERE for that CPU.

If my guess is correct and you have a 915G chipset, the CPU will not work. Not sure what it would do, but it wouldn't be worth 45 pounds.

Yeah. Got the 915G/915P chipset, pentium 4 530 3.0Ghz processor and a Nvidia GeForce 9600GT 512mb graphics card. So it's not looking good on the dual core front. I think I'll just wait a bit and buy a laptop...  

But at least finally sorted out what I can and can't do with this mobo. Computers are bastard complicated... Cheers for all your help friendly hardware gurus!

No, it's just LGA775 with 3 or 4 mutually incompatible generations. Any other modern socket, you can use any CPU designed for the socket, and some are even backwards compatible (notably AM2+). Only exceptions are a few top-of-the-line 140w CPUs that exceed the power limitations of cheap mobos.

Laptop's always good if you want the mobility (I know I do). A note of caution if you decide to make a new desktop build. For a period of time (which ended about 2006), Dell used a proprietary arrangement of pins on some its PSUs and motherboards. If you have a stock Dell PSU, this means that your PSU may not work in a new desktop build. Pretty much all you can bring along is the 9600GT.