This is a very good question - shows some good thought and planning. I don't think Netflow will overwhelm your 10Mb link alone, but it will have to share bandwidth (b/w) with user traffic. I would first look at your peak and average b/w is now and historically and determine how close to full the link is. If you are not running topped out then you will probably be OK. I suggest adding Netflow to only one IF first (sounds like you may not need more than 1 IF) on off peak hours start collecting and monitor the b/w. One strange feature of Netflow, as a technology, is that it does not collect Netflow as a traffic type, so you can't use netflow to measure netflow.
This is just my experience, so caveat emptor...
If you have the IP route flow cache enabled on the router interfaces, you can get a rough idea of the data rate for NetFlow this way... To see and estimate of the current number of flows.
RTR01#sh ip cache flow
IP packet size distribution (7098M total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.001 .445 .157 .028 .028 .028 .095 .012 .007 .004 .003 .001 .002 .002 .001
512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.001 .005 .004 .010 .156 .000 .000 .000 .000 .000 .000
IP Flow Switching Cache, 278544 bytes
3522 active, 574 inactive, 571918570 added
3796396191 ager polls, 0 flow alloc failures
Active flows timeout in 30 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 124104 bytes
3524 active, 1596 inactive, 571925955 added, 571918570 added to flow
0 alloc failures, 12317 force free
5 chunks, 12225 chunks added
last clearing of statistics never
lots more stuff from all the flows...
With this number of say 3500 flows as a reference, there are about 30 flows in each UDP packet in my case. It seems to update about 8 UDP flow packets per second so that is ~12Kbytes/second. a 10Mbps pipe is ~1250Kbytes/second so Cisco's reference estimate of ~1% of the pipe traffic seems spot on...
One more thought... as you enable the flow collection put a packet analyzer on the traffic headed for the NetFlow collector (the default UDP port is 2055). From that you can see exactly what traffic is using the WAN link for NetFlow. If I capture during a fairly busy time (right now), I see about 8 packets each second...
This will eliminate any guesswork...
NetFlow exports are typically 1-2% of the traffic they describe. So on a 10MB link, you probably won't even notice it.
This is true. Because NetFlow collects on ingress only it can not report on any traffic created on the source, but you can see that traffic downstream. I'm not advocating a string of collectors pointing to NTA, just a method of determining NTA B/W usage on a particular link.
NetFlow data collection is both Ingress and Egress though there are some restrictions with either of course. Many routers support Egress NetFlow after 12.3(11)T...
One point to note, to my knowledge Netflow statistics can not be generated by a 3750. We use a number of them and have fairly recent code and do not see it as an option.
Here is a document that provides more detail on NetFlow, platforms and such.
Here is one that coveres the platforms and commands required with examples for most platforms...