If you went intel and needed to stay in the $200 budget for mobo and cpu then I'd look the Fry's ad if you have a local Fry's because they will have an E6550 and mobo for $159 on a pretty regular basis. Granted the mobo sucks, the CPU is great and you are essentially getting the mobo for free.
Get an E21X0. They have low clocks yes but they are cheap and can overclock like monsters. I settled for an E2180 for 70 bucks and I got it running at 3.2 GHz up from a stock of 2.0 GHz. Sure the cache is only 1 MB but if you go look at some charts comparing performance levels with cache size, it is a fairly negligible blow. If you have no problem spending up to 200 bucks plus or minus a bit you have a lot of options. At the top of your price limit you can get an E8400 which is a Core 2 Duo at 3.0 GHz plus its 45 nm. However if you jump up a little bit by about 15 bucks you can get a Q6600 which is a quad core. The downside to the quad core is it is 600 MHz slower than your other option but If you are ok with overclocking you can easily push it to the same clock speed on the same FSB. If you want to save some money you could go E6850 which is the same as the E8400 except its 65nm rather than 45nm. If you want to keep the cost at about 100 you could get the E2XXX or E4XXX series. E2XXX is what I chose because the rest of the computer I built cost so much (I'll eventually throw a Q9450 in there). If the E2XXX series's 1MB cache bothers you you can go for the E4XXX which has the 2MB Cache for about 20 bucks more. It's really up to you. You really have to consider how much cache size really matters to you, wether you plan or are comfortable with overclocking and wether or not you want dual or quad core and to top it off if you are willing to save money.
Indeed, I dont know what motherboard you have, but I would defiantly look at OCing what you have and wait for Nahelem if your gonna upgrade to Intel. The new arch is gonna require all new everything, so Skt 775 is nearing the end of upgradeability. The flip side of that is that the Core 2 is pretty powerfull and should last 3-5 years for many users. I guess it all depends on how important the $$ are to ya.
I would OC as well for now. It would have been real nice if that 5000+ was a Black Edition cpu.
I would make it last a few more months until all of the new releases have started to surface. It should open the door to the opportunity for an upgrade to something upgradable for a much longer period or it should drive down the prices of some of the current cpus.