|The LE-1250 is a socket AM2 processor, not 939. |
The original Phenom X4 processors would work on certain AM2 motherboards, but based on the information I found on Dell's support site, your board only accepts K8-based CPUs. The fastest K8 processor is the Athlon 64 X2 6400+. However, that's a 125w CPU, and a Dell board might panic and/or melt if you attempt to run such a power hungry processor. Instead, go for the 89w version of the X2 6000+. It has two cores, both of which run at 3.1GHz, and a total of 1MB of L2 cache. That would be a huge step up from your Sempron.
Super PIII | Unlocked ES Tualatin @ 1.8GHz (150x12, 1.65v, 512K L2)
2GB PC2700 | 500GB | Radeon x1950Pro | Apollo Pro 266T | Win 7 Pro