Ok, well, see, if it's an old 810 chipset in a dell, you're likely limited to a non-Tualatin 66 or 100 fsb (dell doesn't include overclocking or special dividers) unless there's the magic 1/4 divider for your PCI bus. That's unlikely. Assuming your celeron is a 100 MHz FSB Cumine (I'm assuming this isn't a slot 1 based system, though it could well be and you've not specified if it is or is not) cores only went up to the P3 1.1 GHz (I have that in my snapstream rig). This would be a healthy upgrade, but these CPUs are very difficult to find. You might need a socket 370 to slot 1 adapter card, probably need a new Heatsink as well, and we cannot tell you if your motherboard actually can be set for a 11x multiplier that this type of cpu would need. You'd end up spending about $100 between the HS, chip, and the paste, and perhaps a slotket adapter card. You should figure out if it's an 810 or an 810 e though, cause the e series did allow 133FSB. Also, 810 chipsets may support tualatin cores, depending on if the bios and manufaturer set your motherboard up to do so.
You would also get a boost from more memory... but not a night and day difference.
Now, the more difficult trick of upgrading these older systems is to wrap tiny wires around certain pins, insulate other pins, and then attach Tualatin core P3/Celerons to these older chipsets. There's many threads about that on this board and others. You'd have to worry about your maximum divider (again). Also, Tualatin Celerons were 100 FSB with a 256 k L2 cache while the Tualatin P3s were 133 FSB (which I don't think your motherboard will support) with a 512 k L2 cache. This would be faster than the P3 1.1 upgrade, but we're not getting beyond P4 2.0 ghz performance levels.
Your ATI 7000 is a good PCI card, I have one. Going higher than that on the PCI bus isn't going to be a big improvement. Your PCI bus will get hosed with graphics, disk access data, etc that will not do much to improve your RTCW playing ability.
So, yeah, maybe do a re-install of the OS so things are squaky clean, but your dell has very little upgrade potential. You have my sympathy, I'm in a similar dead end with my Celeron 1.3 and P3 1.1 homebuilts. I'm going to build a 800FSB intel system soon to combat this.