First...what do you want your PC to be able to do that it cannot do now?
Next...what does the inside of your pocketbook look like?
Quite often you'll find that upgrades cost too much and don't provide that much more in ability.
Intel Pentium E5400 @ 2.70GHz 44 °C
Wolfdale 45nm Technology
8.00GB Dual-Channel DDR2 @ 399MHz (5-5-5-18)
XFX MB-750I-72P9 (CPU 1) 39 °C
1024MB GeForce GTS 250 (XFX Pine Group) 60 °C
932GB Western Digital WDC WD10 02FAEX-00Z3A SCSI Disk Device (SATA)