Yogani,
First, the index.dat file, as Ruirib says, is used by IE, and shouldn't be removed, mine is 16 MB too, and shouldn't affect performance. It's the multitude of the other Snitz files that will make it run slower when they build up to large amounts. So, I think it's best to work as you have done before, to delete them regularly. And you can of course test if one of the larger limits work better than the other, like 1 GB, as you had before, or 5 GB as you set it now.
I tested it on one more computer with Win98 and IE6, no temp. files generated by the forum, so the Snitz folders are created because you administer the forum. And the temp. main folder is named \\Content.IE5 here too, that's obviously what IE6 uses.
There is a good chance that performance would be better also if you upgrade the RAM to 512 MB. You can test how much RAM is currently used by running the utility System Monitor, under Start > Programs > Accessories > System Tools and then select "Add Item", under Category "Memory Manager" select "Allocated memory". If you run this utility when things slow down and Allocated RAM would be above your physical 256 MB, then obviously a memory upgrade would help because the harddrive as virtual memory is much slower than an additional RAM chip.
If you don't see System Monitor under Accessories > System Tools, go to Control Panel > Add/Remove Programs > Windows Setup > System Tools, highlight it, click the button Details, and put a checkmark on System Monitor, then click OK on all windows, and the utility will be installed.
Another useful utility is "Resource Meter", showing Windows Resources, if they go down below 15% or so, then everything will come to a crawl and Windows may even freeze. Windows resources are mostly used up by having many windows open at the same time. But if you find that running the forum itself would use up the Windows Resources, then the issue is Win98 itself, which handles these poorly, and only an upgrade to Win2000 or WinXP would take care of that.
I think that long load times that you may experience vs. other "regular" users are unrelated, yours because of the temp. files, and others temporarily by traffic or issues on the server, and of course their own internet connection.
