Ha. What a dé-jà-vu. I remember everything you mentioned. The impossibility to change weapons, the desperate looks to the ground just to raise fps. And in one moment you got 40 frames just for them to drop to 9 in the next second. The only good thing was, that I was always warned when enemies appeared. The fps drops were treacherous.
Ok, you want hints, right? I barely can give any. I concentrated on actions that did not bring me into close combat brawls. I secured rear areas behind the line. Rearward spawns with only little enemy activity as it is the appearance of enemies that cause your fps to drop. Usually I was watching some rear flag and when a single enemy or bunch of enemies appeared, it was still less enough for the fps to stay stable. I also liked to operate arti which is usually behind the frontline. It might stutter a bit when you shift to the spotter view, but overall it doesn't get you killed right away. I became really good with the Nebelwerfer in those days. And, well, stay out of knifefights.
In the end I had some mindmap of spots to go and no-go-areas on each map. Many maps provide spots with less combat activity which are non the less very important to keep in order to win. There were barely maps where I did not found such places. Though you might need the lonewolf gene to enjoy.
However, don't let it demoralize you. In a knifefight you turn 180 degrees right away because you know your computer won't deliver a frame at 90 degrees in time so you just catch your enemy at 180 degrees. It takes a lot to compensate fps drops and actually kill people when others turn around like cutting through butter. This will not be in vain. When you get a new rig, you'll benefit from your expirience you've made here. The things you've found out about lonely spots behind the line will reward you with a 7th sense for enemy appearances and breakthroughs.
Another example: I'm using arrow keys and besides the unability to change weapons the keys were too far away anyway. The knowledge I gained affects my behavior today. I still don't switch to knifes in a supposed knifefight, I simply shoot the guys.
What I consider unbelievable is that you still play on 512MB RAM. You've somewhat missed several phases in RAM prices here. At first the RAM you need was very expensive, became very cheap while prices rose again after some time as it is barely produced or produced in very small amounts nowadays. I guess you're using DDR266 or even DDR400 if it is an early Pentium 4 system.
FH1 is very hungry for RAM and requires at least 640MB Ram to run smoothly. A simple upgrade would have solved the worst of your headaches already years ago while it might only have been 100 bucks you would have needed. Of all upgrades I did through the years I played FH, the RAM upgrade gave the best cost-performance ratio.
If you're going to spend a lot more time on FH1 servers until you upgrade your rig, consider to upgrade to 1GB RAM. I'd even do that today if the new rig would still be 6 month away.
Download something like
SiSoftSandra and check what RAM you're using. It might be that you're using the RAM in a dual mode channel which reqiures 2 banks actually to be equiped to work correctly. Then it would be 2 banks occupied with 256MB each. Old computers like yours often use an outdated bios that cannot handle 2 gigs of RAM. So for you it would be suitable to use the same amount of banks used currently and that means that you would have to exchange either a single 512MB stick or or 2x256MB sticks. Accordingly I'd recommend to double the memory size you're using currently. If there is a 512MB stick assembled buy a 1Gig stick. If it is 2x256MB buy 2x512MB. Then it is most likely that you won't mess anything up. A 512MB DDR1-400Mhz stick is currently about 20€-30€ here, shouldn't be that expensive. DDR1 memory is compatible downwards, so it doesn't matter if your mainboard is running on 266Mhz, 333Mhz or 400Mhz, 400Mhz DDR1 sticks would do.