Wait or replace (E8400).

Your E8400 OC'd to 4GHz is likely just barely enough to make your HD5770 break a sweat so you would only get a modest improvement from getting a R9-270 before upgrading your CPU. You can open the Catalyst Control Center or equivalent utility to keep an eye on GPU load to see if your E8400 actually manages to max out your HD5770. My own experience with a stock E8400 and HD5770 is the CPU was not fast enough to push the GPU much beyond 70% load so, at 4GHz, you might barely manage to bottleneck the GPU some of the time.

You would probably get more benefit from upgrading your CPU/MoBo/RAM first.

As for the next wave of CPUs, we are talking ~6 months for Intel's Broadwell or 12-18 months for Skylake.

I would not bother trying to "match" a CPU and GPU since the "ideal" balance between the two chances on a game-by-game, scene-by-scene basis. Instead, I pick upgrades based on most bang-per-buck for the foreseeable future considering the immediate benefit in my current system and reuse value once combined with planned future upgrades.

The CPU/MoBo/RAM upgrade going from an E8400 to i5-3470 makes gaming and general computing a fair bit more pleasant. I'm still using my HD5770 - still plays everything I remotely care about well enough to hold at least until 20nm GPUs come out. Whatever GPU I end up buying then will most likely carry over to my next PC 2-3 years from now. Tick-Tock.

Right, in order to get bang for the buck to make sense, you need to have parts that match up performance-wise pretty well, especially for very demanding games such as skyrim with mod packs. There are tons of products that are tremendous bang for the buck, but unless you pair it with something that can compliment it, then you're not getting bang for the buck.

Obsessing over getting the absolute best balance under every conceivable scenario is not particularly healthy since it is impossible to achieve. I made my peace with that years ago and only upgrade what needs upgrading when it needs upgrading, taking into account potential reusability. For me, there is no value in replacing what is not broken.

If I had wanted the "best match" back when I was using my E8400, that would have been a HD5670 for ~$120 but for only ~$20 extra, I got a HD5770 which is nearly twice as powerful. If I had gotten the HD5670, I would have started to itch for an upgrade after two or three years but with my HD5770, I'm about five years in and still do not feel any (legit) urge to upgrade.

Roughly doubling the GPU's useful life for an extra ~20% up-front cost even if the GPU would be considered a mismatch on both systems I used it in by many people here (too high for the C2D, too low for the i5), it still gave me terrific value over its useful service life since I had planned for it to span at least two systems.

I agree with speccing parts with longevity in mind, I've got a lot of systems around for a makeshift render farm, I think we are on a similar page, I definitely understand the value of the minor price step up for major performance gains. There is a definite sweet spot though. In no way am I saying a perfectly balanced system. To me it just makes very little sense to buy a part such as a GPU when the platform cannot take advantage of it and then down the road when you do buy a new platform, the GPU which you bought before obviously has the same performance levels, but now, the money from the GPU (which was wasted on a bottlenecking platform) could buy you something three times as powerful.

Yes the GPU seems to be the more often needed upgrade if the user keeps up with modern and demanding titles, but in some cases, the platform should definitely come first since the upgrade cycle for a platform I think is ~5 years while the GPU (depending on anticipated games played over the course of the intended life cycle) seems to be every 2.5 years. -- Again, not a perfect balanced system, but components that aren't bottle-necking each other, the money which could be saved to buy a part at the same price point when the bottlenecking part(s) can be upgraded as well.

Not quite... and three times as powerful is a bit optimistic since GPU progress has slowed down considerably over the past couple of years: the R7-260X is just over twice as fast as the HD5770 which occupied roughly the same market segment almost five years ago.

If you buy a $100 "just right" GPU for your old system and bury it when you get a new system and a new $100-150 GPU, you have now spent $200-250 on GPUs. If you bought a somewhat overkill GPU for $150 for your previous system and carried it over, you have spent $150 on a single GPU and have $50-100 extra left to go slightly overkill on the next one later, when your carry-over GPU no longer feels adequate and faster GPUs have become available at similar or lower prices.

Also, even if a component is somewhat overkill for a given system, you always have the "outlier" apps and games that will more heavily stress that component than the other weaker components under at least some circumstances and when the previously installed component was a significant bottleneck, the overkill upgrade will still yield a substantial improvement even if it the rest of the system can rarely make it break a sweat.

It all might even up eventually but by not spending/upgrading until I actually need to, I do get much greater bang-for-my-buck than upgrading everything at once every time and then have to worry about getting it right and software/games throwing curved balls by being unexpectedly harsh on one component.

Another benefit of upgrading components as-needed is it simplifies buyers' remorse: the upgrade was overdue and now it is taken care of for the foreseeable future, end of story.

As for OP's case, he needs a new CPU more than he needs a new GPU since his E8400 is already somewhat under-powered even for his current HD5770. If he got the R9-270X first, the 270X would hardly ever get a chance to break a sweat before OP can afford to upgrade the CPU and by then, the R9-3xx will probably have launched. The CPU upgrade is also a safer investment since prices and performance have been practically set in stone for the past few years and are unlikely to change much in the foreseeable future.

Higher quality boards often have more filter caps, higher quality caps, inductors with beefier wiring, more efficient and more stable VRM, etc. that all can help with reaching higher stable overclocks.

On the other hand, if you buy an i5, you already beat heavily overclocked AMD CPUs in practically all games and most applications without overclocking. With modern Intel chips only having ~15% overclock headroom, you are not going to get anywhere near as much overclocking bang-per-buck out of those as you did with your E8400.

It may have eight cores but most games, particularly older games, are heavily dependent on single-threaded performance and for those games, you need to overclock AMD chips to ~5GHz to match stock Intel performance and at 5GHz, AMD's chips use 150-180W more power than Intel's and to get to 5GHz, you would spend about as much on OC-oriented components as.the price difference.

Of course, older titles are usually not as processing-intensive. That said, most people do not care too much about the i5 being able to hit 100+ fps in a game as long as whatever CPU they have can manage a steady 60 fps to match their display's refresh rate and as far as that goes, the FX63xx/83xx usually get the job done even without OC.