A couple of things to add... just becuase you have win98 doesn't mean you can't take advantage of 64 bit processors, microsoft hasn't released "longhorn" which is it's new 64 bit platform, and probably won't for quite a while. On the same token, there won't be any 64 bit programs out there for quite a while probably... so nobody is really in the position to take advantage of the 64bits at the current moment (you can have a lot more RAM in a computer now though).
So why get the AMD 64 bit processor? Becuase it's one helluva processor basically. Takes intel's pentium 4's to school in everyway possible... the frontside bus is amazingly fast, the overheating problems AMD's where known for in there t-bird line of processors and have had people leary of sense are virtually gone, and it's fast... really fast. It's simply an excellent chip, that can really help any 32bit computer, whether it be win9x (95,98,ME) or winNT (2000,XP).
There are some really nice motherboards coming out for the 64 bit processors too, if you are looking into buying a new computer or upgrading in the next few months... i'd definatly recommend going the 64bit AMD route. the new nforce4 boards will be out soon... and they have everything.
AS for Intel, well, dual core is definatly going to be what we are going to see in the next generation of intel's processors probably. The problem with dual cores, or with dual processors, is that you can never get 100% performance out of them. Most applications don't take advantage of them, and even the ones that do... well, there is still overhead with each processor in getting the commands through and in the right order. So if you have two 1ghz processors, you arn't going to get 2ghz of raw processing power out of them, some of that will be taken up in overhead in keeping each processor in sync and figuring out which instructions go where... you might get 90% of that 2ghz for your app if it's a well coded multi-threading application.