Very well, this is enough. No need to post the result of that command. Can find specs easily with that info you just posted.
Ok, so the processor's score is around 300 points here http://www.passmark.com/cpubenchmark/cp ... GHz&id=642
which means that it's as powerful as my netbook's Atom N455 (score 280-ish), with the marked difference of consuming something like ten times
as much power (or twice as much as my whole netbook). I say it's not going so bad here on my netbook with LMDE and XFCE, it's of course weak but does not lag when doing office stuff and internet browsing (can't play flash videos beyond 360p, but can play non-HD videos fine if in the hard drive). But I'm using a decent sata drive and 2 GB of ram.
Your motherboard seems to be available in two revisions in the manufacturer's site, 1.something and 2.0. Check the numbers printed on the board to see what revision it is and also look at what bios version it has. Depending on what revision it is, you can get an actual pentium D or just Pentium 4 with hyperthreading (a trick to disguise themselves as dual cores, better performance than being single core, worse than being actually dual cores).
In any case, look up the cpu you are looking to get in that site I linked above to get a rough estimate at how much better it is than what you have now. In most cases, performance should increase noticeably.
Also, make sure that the new cpu supports 64-bit operating systems (for future-proofing and to try out 64-bit linux distros), and that the TDP (thermal design power, or how much heat it generates) is close or the same as the cpu you have already otherwise you may need to upgrade the cooler too. For example the Pentium 4 661 is ok, any Pentium D is close enough, the pentium 4 670 is too high.
I modded a bit the cooler on my old Pentium D to be able to cool it with the air pushed by a 20cm-diameter (thus very silent) ducted fan I had around, although now I cannot close the side of the case lol. Just to say that if you don't mind ghetto solutions, it's doable to run the PC in near silence or upgrade the cooling system without buying an actual high-end cooler.
However, there are a few AGP cards still available at reasonable prices.
ATI abandoned AGP interface a bit later than Nvidia, so you can find more powerful cards with OpenGL 2.0 and even native HDMI ports. Also ATI card opensource linux driver (the driver integrated in kernel, see here https://wiki.debian.org/AtiHowTo
) is better than the one for Nvidia cards (that are supported better by the official nvidia legacy driver).
Now, there are companies that do weird things like making PCI cards with relatively new nvidia entry-level chips, but none I've seen can be said to be cheap.
I'm surprised that the SATA drives outperform EIDE by as much as you mentioned.
Well, it's not because of sata per-se, that's just an interface. It's because the hard drive machinery of a drive with sata interface is more modern, and has better performance. Also having a dedicated port and cable tends to help when both drives are accessed at the same time.
What matters most for system responsiveness is, like you said, small-medium file write/read and also seek times/latency.
All these things are mostly dependent on the actual hardware inside the drive. The newer it is, the better. They stopped making IDE drives long ago, so they are worse only because they are older, not because of IDE.
The sata ports on that motherboard are sata1, so the bandwith is more or less the same as an IDE port (max 133 mb/s). Point is, this will bottleneck newer hard drives only when moving very large files. And even then, not by much. IDE drives rarely reach 50-60 mb/s of large file transfer rate.
The random read/write speed of small-medium files on a sata drive will be MUCH higher than any IDE drive you can find, but still not anywhere near the max speed of the sata1 interface so you get a responsiveness improvement without bottlenecking.
Sata2 has twice as that much bandwith, Sata3 has four times as that. But a conventional hard drive is already more or less maxed out on a Sata2 interface. Sata3 exists mostly for SSDs that can actually use so much bandwith.
Sata2 hard drives can be installed on a Sata1 motherboard and either adjust automatically or have a jumper to set them to Sata1 interface, Sata3 hard drives should do the same but I never tried personally.