Alas not so, a 7870 would easily be bottlenecked by a 4000+. I've benchmarked this extensively using
a 6000+. A 4000+ couldn't even properly exploit an 8800GT, never mind something as recent as a 7870.
See my results:
http://www.sgidepot.co.uk/sgi.html#PC
Honestly, don't put in anything stronger than a GTX 460 because it'd be wasted on a CPU
of that era, and I only mention the GTX 460 because they're available so cheaply on eBay
these days (even I have a whole bunch to sell off).
I used to have a 6000+ 3GHz with an 8800GT. I added a 2nd for SLI. Later I moved the two
cards onto a newer mbd/CPU (i7 870), performance doubled compared to using the same cards
with the 6000+. Ordinarily one might say look out for a Phenom II, but alas your board does
not support any Phenom/Phenom II CPUs (batmanbob, msi.com shows no Ph/Ph2 in the CPU
support list for the Neo V2).
I notice the 4000+ has a pretty low clock aswell (2 or 2.1GHz depending on whether it's a
Windsor or Brisbane), so oc'ing it would help a bit, though not much really overall.
TBH, it'd be much more productive to just upgrade the whole platform. If you don't
want to buy new, there are plenty of used bargains, right across the performance
scale. Compared to what you have atm, an M3N with a used Ph2 965 and a couple
of 460s or 560s SLI would be a huge speedup, or a single used GTX 580 1.5GB
would be about the same cost as a couple of 560s. Performance would be
quicker with an Intel build, but such parts tend to cost a bit more on the used
market, eg. i5/i7 CPUs, though strangely one can get P67 mbds for low amounts
now, or even Z68. If you really want to step up the scale but still without blowing
your bank balance, get a used Z68 board & i5 2500K, with 8GB DDR3/1600. That'll
be leaps & bounds ahead of the Neo V2 and far more able to exploit the power of
something like a 7870 if you wanted to go that route, or even just with older cards
SLI/CF.
Note that I've been researching these issues for a couple of years now, investigating
bottlenecks in system upgrades. Beyond a certain point, upgrading an older board
isn't worth it. Plus, in some cases, the best CPU for an older board may actually be
surprisingly expensive because it might have unexpected value in the commercial
world for obscure reasons, or there can be demand from collectors, etc. The best
Barton Athlon XP is one such example.
batmanbob is correct in that the degree of bottleneck varies greatly from one
game to another (eg. enormous for X3TC to not much at all in Call of Juarez,
with titles like Stalker somewhere inbetween), but a 4000+ is too low down
the scale IMO to be worth pairing with anything newer than an 8800GT. As I
say, I only mention the GTX 460 instead precisely because they're so cheap.
Ian.
PS. Same applies to PSUs, cases, etc. I keep bagging Thermaltake Toughpower 750W
(or better) when I can, along with Antec 300 cases, TRUE coolers (perfect for oc'ing
any CPU on the cheap), RAM kits, etc. One can do
a lot with a small budget now.
Indeed, one of the toms editors agreed with me in a PM that to some extent buying
new when the 2500K is still such a potent offering doesn't really make any sense.