Is my processor upgradable?

You'd be MUCH better off starting over. You can build a VERY capable computer for $900. Hell $400 on a full setup (minus monitor/mouse/ect) would get you way better performance.

time to build anew

Well the fastest cpu you can get is in this list:

http://www.intel.com/support/motherboards/desktop/D865G...

Celery list:
http://ark.intel.com/ProductCollection.aspx?familyId=52...

P4 List:
http://ark.intel.com/ProductCollection.aspx?familyId=58...

The lists will specify if HT is enabled or not and your looking only for the socket 478 models ... not the LGA775's.

Look for a second hand P4 3.2 / 3.06 which has quite a larger cache which will help possibly.

Note ... check the board revision first ... the processors on the top part of your mobo list are not able to be put on the earlier boards.

You should be able to get one for $10 and be able to push it up to 3.4 easily.

Make sure you have 3 gigs of ram or so which will also speed up the system.

Do the usual too ... disk cleanup / defrag regularly ...

That will give you a bit of a perfomance boost till your cashed up enough to overhaul the entire system ... yes, that's unfortunately the next step.

You might be able to get a better graphics card ... that what do you have now?

Honestly dude there's no point spending the money on a system based on socket 478. You can't get dual cores, the best cards you can get for AGP aren't up to much compared to the mid-range PCI-E cards and would be seriously bottlenecked by the CPU anyway.

Compared to what you're running you can get a massive performance boost for comparatively very little cash.

The OP did say he wasn't cashed up ... hence the advice I gave.

Try reading the post properly - the links stated you can't get dual cores for that board too.

A faster single core 478 cpu and better graphics card (ATI made AGP card up to 3 series) with a stick of ram will make a difference and he will still be able to game at low res.

If you cant add something to the thread then don't post redundant information we already know.

As reynod said, you could find a pentium 4 at starmicro for 35 bucks(3.2ghz), and upgrade to a 3 series ati or up to a 7 series (maybe 6) nvidia, and you should be good to go, then you can save up for a hexa core next year :-)

best solution: cheap computer at local stores, 100-400$ deals and you can manage to get more performance than with your current celeron

you can't easily find any socket 478 anymore...end of life since a while.
use your motherboard site to find processors fitting and google it for availibility.
http://www.intel.com/support/motherboards/desktop/D865G...
Just make sure you dont go or another celeron as its lacks of cache and other features makes it a no go for gaming and high performance usage.

I would suggest you also add ram if you can, it is the poor man upgrade nowadays.
If you are able to find a decent no overprice agp go ahead it would be better than onbaord's