The outside interface on the 501 can do 100 Mbps.
Once you update the IOS to 6.x, the software will enable both interfaces to 10/100.
Here's the output from one of my 501's (Address deleted to protect the innocent

). This is connected to a 3Mbps AT&T DSL I use for inbound VPN to my home network.
interface ethernet0 "outside" is up, line protocol is up
Hardware is i82559 ethernet, address is 0013.6042.8a8c
IP address xxx.xxx.xxx.xxx, subnet mask 255.255.255.255
MTU 1492 bytes,
BW 100000 Kbit full duplex
1476378 packets input, 1086093230 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
1134998 packets output, 98891755 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collisions, 0 deferred
0 lost carrier, 0 no carrier
input queue (curr/max blocks): hardware (128/128) software (0/4)
output queue (curr/max blocks): hardware (0/25) software (0/1)
(Show version)
Cisco PIX Firewall Version 6.3(5)
Cisco PIX Device Manager Version 3.0(4)
Compiled on Thu 04-Aug-05 21:40 by morlee
pix-dsl up 60 days 9 hours
Hardware: PIX-501, 16 MB RAM, CPU Am5x86 133 MHz
Flash E28F640J3 @ 0x3000000, 8MB
BIOS Flash E28F640J3 @ 0xfffd8000, 128KB
0: ethernet0: address is 0013.6042.8a8c, irq 9
1: ethernet1: address is 0013.6042.8a8d, irq 10
Licensed Features:
Failover: Disabled
VPN-DES: Enabled
VPN-3DES-AES: Enabled
Maximum Physical Interfaces: 2
Maximum Interfaces: 2
Cut-through Proxy: Enabled
Guards: Enabled
URL-filtering: Enabled
Inside Hosts: 10
Throughput: Unlimited
IKE peers: 10
This PIX has a Restricted (R) license.
Serial Number: {Deleted}
Running Activation Key: {Deleted}
Configuration last modified by enable_15 at 12:08:20.981 CDT Sun Oct 22 2006
pix-dsl#
FWIW
Scott