Your 5300 is the same config as mine was when it was new, I had a 650 as well. You can go to http://www.pricewatch.com and check for 1 Ghz processors, but since Intel doesn't make this processor any more you won't get many retail hits, and in my experience the prices aren't that great. Much better off to find one on ebay, there's always several listed, they tend to close at less than $80, and I've often seen "buy it now" auctions below $70. Search ebay for SL53S and you'll find them. Be sure and include item descriptions in the search to make sure you get them all. The next lower option is the Intel SL53L, which is a PIII 850, they go for about $35. If you want to go that route I have one I'd sell. Intel didn't make 900 or 950 Mhz processors in the 100 Mhz FSB flavor, so those are your options.
Replacing the processor is not tough. If you decide to try this shoot me a message and I'll provide instructions and pictures.
When you look at the bottom of the laptop, in the bottom right hand corner is a cover secured by one screw, that's the mini-pci slot. If you have an internal modem or combo modem/NIC, it's parked in there. There are three connectors in here; at the edge near the front there is the actual mini-pci connector, the cards insert the same way your SODIMM memory does, that is, into the slot at an angle and push in until it locks in place. The other two connectors are a 2 pin that extends the modem to the external RJ-11, and an 8 pin that extends the NIC to the external RJ-45. If this slot is populated you'll have to decide if you want an internal wireless NIC more than you want the modem or combo card you'll have to remove to accomodate it.
Mini-pci wireless NICs are plentiful on ebay, but I've never seen them for retail sale. Most of the ones on ebay are clearance stock or pulls from working systems. All the options available in PCMCIA form factor are here as well: 802.11A/B/G. You probably won't find any of the bonded channel turbo or proprietary schemes here, these are meant for wide compatibility. For whatever reason you almost never see the mini-pci wireless NIC for sale with the antenna, which is a shame, because this is where the advantage of the internal wireless NIC comes in. The antenna consists of two wires about 1.5 feet long each, with tiny gold coax connectors on one end each. Once the NIC is installed you need to open the laptop case again. Connect the anbtennas to the coax connectors on the NIC, then wrap them around the perimeter of the inside of the case. I used RTV to secure mine, but super glue would do as well. This NIC is now providing dual space diversity on the receive antenna, which is the same way most WAPs and wireless routers transmit. My signal strength is through the roof compared to my old Cisco card.
The problem is probably environmental. The antennas included with most WAPs and wireless routers are middling devices at best. You could be getting interference from wireless phones, or possibly even a neighbor's router, especially if your neighbor is running one of the god-awful proprietary channel bonding turbo schemes. A quick fix would be to see if changing channels helps. If so, then try all the available until you discover which one provides the strongest signal. Also try repositioning your WAP or router, if possible. I can't comment on whether these after-market antennas are worthwhile, I haven't tried them.
Again, if you need any help with any of this, let me know here. Good luck.