With a P4 2.4GHz with 256MB of RAM, you can run a lot. Although I would
upgrade the RAM to 1GB and get a fast graphics card, e.g. ATI 9700 Pro.
With the additions I mentioned, you can run almost anything fast, Photoshop,
Unreal, Unreal Tournament 2003. In fact, a machine like that is almost a
server.

Your worry is understandable, considering you are spending money on an
upgrade.

~ Adrian ~