That's a good point actually - the 1.5 might be an old Socket423 Williamette core, in which case you're not gonna fit any newer P4s in there. I think they did go up to a 2Ghz S423 Willy, but there'd be little point in that upgrade.
Another question that springs to mind is do you know what RAM you have in that system? Some earlier P4 Systems were available with PC133 SDRAM. If this is the case in your system, you'd probably barely notice any difference anyway, as the RAM is the real bottleneck.
If you <b>do</b> happen to be unfortunate enough to have a S423 Willy with SDRAM, then you'll have to junk everything to get much performance increase, really.
---
$hit Happens. I just wish it would happen to someone else for a change.