This deal seems almost too good to pass up:

http://www.pixmania.co.uk/graphics-card/sapphire-technology-radeon-hd-7790-1gb-gddr5-pci-express-3-0-11210-00-20g/21323666-a.html?ectrans=1&gclid=CM6VzemE7rkCFY_KtAoduV8AmQ#srcid=11270&nopopup=1&merch=1

It's 20% off, and nobody else is selling it for any less than £95.

Some background: I previously had a Palit (ugh) GTX 460 SE, which seems to have been taken out by a failing PSU, a few months out of the warranty that I can't find and probably wouldn't want to redeem anyway because I would just get another 460 SE.

The 7790 is a step up, but it's not as good as the 7850 (about £30 more) or even the GTX 650 Ti Boost (but that seems absurdly large and I worry it wouldn't even fit in the case - there's about 37cm available from what I can see).

Is it worth going for the deal, waiting for something better, or doing something else?

ahhh a bonaire model, it's not that great and not a big step up from GTX 460, but I suppose it does the job if ur playing on lower resolutions. I wouldn't get it though myself in your situation and would just get a 7870 from the cheapest place possible. That card is really good value. Or possibly wait for the R7s/R9s. (I'm actually looking at the cheapest R9 atm for my next build some time in the next few months)

Though what other parts do you have, will feel more comfortable giving advice if I know about possible bottlenecks and sorry to hear about your PSU. It wasn't a quality brand?

It was a Xigmatek, which I thought was good but apparently isn't.

Athlon II X4 which is holding up remarkably well. 4gb ram with 4 more on the way.

Yeah most games dont push multicores to the max yet, but we're slowly getting there. Faster thanks to next gen consoles I hope.

I'd definately pass that offer with your setup
definately go with 7870, in your case if you want to get up to double the longevity out of your setup. I think Sapphire or HiS has some good models going for 150-180£. If you look around a bit you might find some for even less. 7850 is priced in a way that its just not worth really going for it over the 7870.

http://gpuboss.com/gpus/Radeon-HD-7870-vs-Radeon-HD-7790 some comparisons

Anandtech's Bench is better to compare cards.
http://www.anandtech.com/bench/GPU13/583

I agree with Vashyo, grab a Radeon 7870, it's got great price/performance.

Or, wait for Volcanic Islands to launch, majority of that line-up should be re-branded cards, even still it should bring with it some price drops on the 7000 series.

Also, Isn't that Athlon 2 x4 getting a little long in the teeth? Thought about picking up a Phenom 2 x4 dirt cheap off ebay or something for that extra boost? Or even a Phenom 2 x6. :)
Then sell your Athlon 2 x4 CPU which could go towards paying for the Phenom!

The 7870 is getting up to twice as expensive as the 7790 though.

Roughly when will the the 8000 series announcement happen?

As for the CPU, I'll leave that for a future upgrade. Is there anything I need to be concerned about motherboard-wise (it's AM3+) or will any AMD CPU be fine? Power isn't a concern anymore, the new PSU is 750W.

The 8000 series was released a long time ago, OEM only. Aka. Dell machines which were re-badged 7000 series of cards.

The new series is named like: R9 290X

Yeah, the 7870 is more expensive, but it's also far more powerfull and is worth it from a price/performance perspective.
If you can find the middle ground like the 7850, go for that instead, but only if it's significantly cheaper.

That's highly dependant on the motherboard in question, in theory you could drop in any Athlon 2, Phenom 2 or FX processor.
However some motherboards have power limits, for example maximum 95w TDP CPU's, so check your motherboard on the manufacturers website to see a list of supported processors.

Also, the PSU being 750w says little about what the actual power the PSU could deliver, for example I've seen Shaw 800w PSU's literally catch on fire even under a 200-400w load, providing a brand and model name would go a long way. :P

Corsair CX750, and if I know one thing about component brands, it's "you can trust Corsair".

Will I still see benefit from a 7870 over a 7790 even if I stick with this Athlon II X4 640?

It is but the R series thats gonna be released in October just has soo much damn performance over this generation... the R9 290X can beat the Titan so I am sure the R7 270X (which should cost like 150-200) can probably beat a 670 or be on par with it...