If the P4 2.4 is a B model (533 bus) it's slower than your AXP2800+. If it's a C model (800 bus) it is about equal. You could probably OC his 2.4 to near 3.0 GHz.
Honestly, if you are gaming on this machine, any CPU you can put on either motherboard is quite low for the newest games. I think it's a waste to buy a CPU for either system. You would be much better off with a budget socket AM2 or 775 dual core setup instead. You will need a mobo, cpu, DDR2 memory, and a PCI-e video card. Also possibly a new power supply depending what your currently available ones are and the video card you choose.
Anyway, sorry to bare bad news. There are plenty of old games you should be able to play. But If you hope to play the latest and future games, you should aim toward a dual core system within budget. I was trying to play the new Race Driver: Grid demo yesterday on an A64 3700+ (once mighty single core) and it struggled under 30 fps most of the time on low details even at 640x480 resolution.
Edit: I assume you have an Athlon XP 2800+ and not an Athlon 64 2800+. The A64 2800+ is much faster than the Athlon XP or the P4. Still single core though.
I don't pay games`at all.. however i do do some video editing but ainly photo editing in PhotoShop. I use Nikon NX which is slowish.. so maybe a new system is a better way forward.
You say get a Dual Core CPU.. this will have to be Intel based as my Video editor sw prefers intel although it did run acceptably on my AMD system. What DualCore CPU do you think is equal in performance to what I have, so I can decide on what I can afford that will be better? Thanks!
Alot depends on your budget and whether you have to use your old memory. There are plenty older model cpus, but few motherboards left for socket 478 and 939 (using ddr). Check starmicro for any oem cpus you're interested in, and computer geeks for older motherboards. For a cheap upgrade using your old ddr, 3btech has an asrock dual vsta socket 775 board for $33 shipped with no accessories (you can get a backplate from asrock america for $5). For the cpu, the 2xxx series core2 duo is priced right, and I use the "pin mod" technique to overclock.