[freakshow]

I don't think you'll be able to upgrade a P4 to make usable for SC2, maybe on the lowest settings.

Here are the min reqs:
PC Minimum System Requirements*:

* Windows® XP/Windows Vista®/Windows® 7 (Updated with the latest Service Packs) with DirectX® 9.0c
* 2.6 GHz Pentium® IV or equivalent AMD Athlon® processor
* 128 MB PCIe NVIDIA® GeForce® 6600 GT or ATI Radeon® 9800 PRO video card or better
* 12 GB available HD space
* 1 GB RAM (1.5 GB required for Windows Vista®/Windows® 7 users)
* DVD-ROM drive
* Broadband Internet connection
* 1024X720 minimum display resolution

And even then, at those specs, the graphics and speed will be so crappy, it's not even worth playing.. just shell out a few hundred bucks for a machine that's actually worthy of running SC2. Even my older IBM T60p can't play it that well on normal settings.