I'm wondering how much money the OP would be wasting if this 'very small platform upgrade' procedure is done enough times. I've been on a tight budget before myself, but I would take some convincing that saving for something more significant wouldn't turn out cheaper in the long run, then using it for as long as possible. I went from an Athlon XP rig to a AX4/Ph2 setup because I knew that SC2 was going to suck on the old rig. I also regretted the cost of that decision for months afterwards (when the recession hit my business).
Alternatively, a cheap Phenom II of some description (probably any, as long as it isn't one of the 'e' type energy-efficient - aka slow - processors) would probably speed up games like SC2 without having to throw out kit.
Teaming up an Athlon II X2 with DDR2 (667MHz as well, hrm) isn't really ideal either. I'd also check to see how much that 4870 is holding things back for the games in question. After all, sticking it in a newer system might impair performance enough to render the whole exercise almost moot (or at least nowhere near expectations in terms of value of investment).
SC2 benches, GPU comparison:
http://anandtech.com/bench/GPU12/404
- edit - The 4870 doesn't fare too badly there. I wonder how well it does under load, because my Ph2 960T / 5770 setup lags a bit with the lighting effects on the 'Outbreak' campaign map.
If I were in the OP's shoes, I would check the cost of the following options:
1) Upgrade current system
a) Cost of Phenom II, as high as poss but an X2 would help:
http://anandtech.com/bench/CPU/129 - 7 or so FPS difference there comparing the lowest AX2 to the lowest Ph2. Looking for overclocking potential might be an idea as well, as I understand it SC2 benefits from higher clocked CPUs.
b) Does your board have DDR3 slots as well as DDR2? If so, what can they handle? Check board manual/specs and DDR3 cost.
c) Failing the previous option, if it was possible to pick up replacement DDR2 at the highest speed the board can handle reasonably cheaply, I'd check.
d) better GPU. Check PSU capabilities (which you'll need to do if you ever think of upgrading the graphics card).
An advantage of improving your current rig is that you can spread out the purchases.
2) Tighten your belt, put up with the current performance, and prepare to go for a decent platform upgrade (possibly keeping your 4870, at least for a while).
I'm sure that a Haswell Pentium would beat my Ph2 rig at SC2, but considering that the G850 (according to the chart I previously linked) is 12FPS better than the highest AX2 in the chart, let's say there's a 15FPS difference between your exact processor and the Haswell Pentium. Is it worth it? How differently will it handle it under the greatest load, what if you're still seeing dips down to say 10FPS?
Money-wise option 2 makes the most sense out of my two suggestions, but go for 1 if you're really feeling the upgrade itch on a tight budget.