If it is a Brisbane based chip, keep an eye on your voltages. Temperature starts to spiral out of control around 1.45v - 1.5v if you don't have good cooling. My old 4200+ would hit a wall around 3.0 GHZ unless I massively pushed voltage, but to me it wasn't worth it. I left it at roughly 2.9 GHZ since that only required a small bump over stock voltage. Great little overclockers for the time, those chips were.
As JBski stated, take care in playing with the base clock. HT tolerance will depend on your board, but usually you can get a little boost there if you play with the multiplier. Memory timings will depend on your modules tolerances, but most name brand 800 MHZ DDR2 chips I ran into would handle 850 - 900 MHZ without too much trouble at stock timings.
@ JBski - Haha! Reminds me of when I shoe-horned a setup and PSU into an old Emachines case. Barely had enough clearance to even fit the DVD drive in without it touching the back of the PSU.
It was quite hilarious how that particular build cascaded.
I started out with a Stock re-manufactured Acer Aspire, and then popped in an HD2800 pro so I could play steam games.
Being thoroughly unimpressed with that card, I dumped it for an HD2900 Pro, and then hit around 7k in 3d mark 06. Wanting more speed, and knowing enough to know that the stock Acer OEM board locked me out of overclocking, as well as the fact it did not have a full PCIe x16 2.0 slot, I replaced the board with a 780G Gigabit board, and picked up 4GB of OCZ reaper DDR2 to go with it. Knowing the stock cooler was just about useless for what I wanted to do, I shopped around for the biggest, baddest air cooler at the time, and then did some crude measurements and figured out that I could squeeze in the Core contact freezer 120. I ran that @ around 2.9GHz for a few months, and then replaced the 4200+ with a 5400+ BE when it came out. Lucky for me the old Acer oem stuff covered the cost of the new cpu (who knew that someone would've wanted the crap oem board) and I ran that one a couple hundred of MHz faster until a couple of years of abuse finally took out one of the BE's cores.
I still run the factory Acer IDE DVD burner in my current system, and recently replaced the 400GB WD drive that came with it.
Now I've got the parts from my old system gathering dust for a HTPC build.