Your main problem is 256mb of ram and the 5200 video card.
You'll want a minimum 1gb ram, 2gb if possible and the best AGP video card money (your money) can buy.
Without a ram upgrade, the main improver, the video card, won't be able to do it's job correctly because your system is so severely bottlenecked by lack of ram.
The CPU is the least of your worries at this stage, though it's still a bit on the weak side, more on that in a minute...
The 3.0 and 3.06 are CPU speeds in GHZ. The speed is derived from the CPU multiplier times the Front Side Bus speed.
CPU multiplier = 23
FSB speed = 133mhz
23 x 133mhz = 3.06 ghz
Or.... if the CPU was a P4 3.0c HT, it would look like this because the C model uses a different bus speed (not compatible with the other type)......
CPU multiplier = 15
FSB speed = 200mhz
15 x 200mhz = 3.0 ghz
Beware and get the correct cpu for your system, the person with the 3.0 may have rounded the speed off.
What you need to know is whether the CPU is a 533mhz FSB or 800mhz FSB speed cpu. I'm not up on all the Intel CPU's of that period, but there may have been other variations as well.
You need to determine exactly what family of processors your PC will accept and go from there.
A better CPU with 2gb of ram and a 7800GS (or above if they start making better AGP cards) will certainly put you in the ball park for FSX, but further upgrading after that may not be possible.
If I were you I'd be inclined to get the Video upgrade, ram, cpu in that order, or skip the CPU and overclock the one you have, that will give you a good indication of how a faster CPU will perform, and indeed, may be good enough without needing to buy a new cpu.
Consider the "core" components only when thinking about an upgrade.
You will need a new:
Video card
RAM
CPU (pending choice not to overclock the present one)
PSU (probable upgrade to power the new video card)
Consider what a new PC really is, it's core components:
Motherboard
CPU
RAM
PSU
Video card
Now consider that the difference between the 2 above options is only a Motherboard and the (possible) second hand price savings from gear bought at ebay.
Price the new components on Ebay or a trusted big online supplier, you may be surprised that a new "core" system isn't much more expensive at all.
Consider also that your current 4-5 yr old technology is nearly past it's use by date, and that money spent on new tech components will bring you up to date and should last you for a few more years.
Here is a cheap but powerful modern mainboard:
http://www2.abit.com.tw/page/au/motherb ... DEFTITLE=YPrice an Intel core 2 E6400 and E6600 CPU (the 6400 can be easily overclocked well in excess of the E6600 with no more than software switches on the Abit mainboard).
Price 2gb of DDR2 PC5300 and the better PC6400 CAS4 G.Skill ram or similar, there are many, many types, and there is always a bargain out there in the fine print if you look closely at the specs.
Price a 7600GT, 7800GT or a 7900GT PCIe video card, the 7600GT is amazing compared to what you have now. The 7900GT is much better again.
Now, discount the PSU as it will be the same cost for both upgade paths.
Compare the "Ebay" upgrade with the "New" upgrade according to cost and there won't be terrible lot of difference.
Compare the two upgrade paths with performance and useable future............... it's chalk and cheese!
There are cheaper new components, the types I listed above are reasonably high end, suitable for a gaming rig.